Provided by: s-nail_14.9.11-2_amd64 bug

NAME

     S-nail [v14.9.11] — send and receive Internet mail

SYNOPSIS

     s-nail [-DdEFinv~#] [-: spec] [-A account] [:-a attachment:] [:-b bcc-addr:]
            [:-C "custom: header":] [:-c cc-addr:] [-M type | -m file | -q file | -t]
            [-r from-addr] [:-S var[=value]:] [-s subject] [:-X cmd:] [-.] :to-addr:
            [-- :mta-option:]

     s-nail [-DdEeHiNnRv~#] [-: spec] [-A account] [:-C "custom: header":] [-L spec]
            [-r from-addr] [:-S var[=value]:] [-u user] [:-X cmd:] [-- :mta-option:]
     s-nail [-DdEeHiNnRv~#] [-: spec] [-A account] [:-C "custom: header":] -f [-L spec]
            [-r from-addr] [:-S var[=value]:] [:-X cmd:] [file] [-- :mta-option:]

     s-nail -h | --help
     s-nail -V | --version

DESCRIPTION

           Compatibility note: S-nail (S-nail) will wrap up into S-mailx in v15.0  (circa  2020).
           Backward  incompatibility  has to be expected – COMMANDS will use Shell-style argument
           quoting rules, for example, and shell metacharacters will become meaningful.  New  and
           old behaviour is flagged [v15-compat] and [no v15-compat], and setting v15-compat, one
           of the many INTERNAL VARIABLES, will choose new behaviour when applicable.  [Obsolete]
           flags what will vanish, and enabling -d or -v enables obsoletion warnings.

     S-nail provides a simple and friendly environment for sending and receiving mail.  It is
     intended to provide the functionality of the POSIX mailx(1) command, but is MIME capable and
     optionally offers extensions for line editing, S/MIME, SMTP and POP3, among others.  S-nail
     divides incoming mail into its constituent messages and allows the user to deal with them in
     any order.  It offers many COMMANDS and INTERNAL VARIABLES for manipulating messages and
     sending mail.  It provides the user simple editing capabilities to ease the composition of
     outgoing messages, and increasingly powerful and reliable non-interactive scripting
     capabilities.

   Options
     -: spec
            Explicitly control which of the Resource files shall be sourced (loaded): if the
            letter ‘s’ is (case-insensitively) part of the spec then the system wide /etc/
            s-nail.rc is sourced, likewise the letter ‘u’ controls sourcing of the user's
            personal ~/.mailrc file, whereas the letters ‘-’ and ‘/’ explicitly forbid sourcing
            of any resource files.  Scripts should use this option: to avoid environmental noise
            they should “detach” from any configuration and create a script-specific environment,
            setting any of the desired INTERNAL VARIABLES via -S and running configurating
            commands via -X.  This option overrides -n.

     -A account
            Executes an account command for the given user email account after program startup is
            complete (all resource files are loaded, any -S setting is being established; only -X
            commands have not been evaluated yet).  Being a special incarnation of defined macros
            for the purpose of bundling longer-lived settings, activating such an email account
            also switches to the accounts primary system mailbox (most likely the inbox).  If the
            operation fails the program will exit if it is used non-interactively, or if any of
            errexit or posix are set.

     -a file[=input-charset[#output-charset]]
            Attach file to the message (for compose mode opportunities refer to ~@ and ~^).
            Filename transformations (also see file) will be performed, except that shell
            variables are not expanded.  Shall file not be accessible but contain a ‘=’
            character, then anything before the last ‘=’ will be used as the filename, anything
            thereafter as a character set specification.

            If an input character set is specified, but no output character set, then the given
            input character set is fixed as-is, and no conversion will be applied; giving the
            empty string or the special string hyphen-minus ‘-’ will be treated as if ttycharset
            has been specified (the default).

            If an output character set has also been given then the conversion will be performed
            exactly as specified and on-the-fly, not considering the file type and content.  As
            an exception, if the output character set is specified as the empty string or hyphen-
            minus ‘-’, then the default conversion algorithm (see Character sets) is applied
            (therefore no conversion is performed on-the-fly, file will be MIME-classified and
            its contents will be inspected first) — without support for character set conversions
            (features does not include the term ‘+iconv’) only this argument is supported.

     -B     ([Obsolete]: S-nail will always use line-buffered output, to gain line-buffered input
            even in batch mode enable batch mode via -#.)

     -b addr
            Send a blind carbon copy to address, if the setting of expandaddr, one of the
            INTERNAL VARIABLES, allows; the ‘shquote’ expandaddr flag is supported.  The option
            may be used multiple times.  Also see the section On sending mail, and non-
            interactive mode.

     -C "field: body"
            Create a custom header which persists for an entire session.  A custom header
            consists of the field name followed by a colon ‘:’ and the field content body, e.g.,
            ‘-C "Blah: Neminem laede; imo omnes, quantum potes, juva"’.  Standard header field
            names cannot be overwritten by custom headers.  Runtime adjustable custom headers are
            available via the variable customhdr, and in compose mode ~^, one of the COMMAND
            ESCAPES, is the most flexible and powerful option to manage message headers.  This
            option may be used multiple times.

     -c addr
            Just like -b, except it places the argument in the list of carbon copies.

     -D     ([Option]) Startup with disconnected set.

     -d     Almost enable a sandbox mode with the internal variable debug; the same can be
            achieved via ‘-S debug’ or ‘set debug’.

     -E     set skipemptybody and thus discard messages with an empty message part body.

     -e     Just check if mail is present (in the system inbox or the one specified via -f): if
            yes, return an exit status of zero, a non-zero value otherwise.  To restrict the set
            of mails to consider in this evaluation a message specification can be added with the
            option -L.

     -F     Save the message to send in a file named after the local part of the first
            recipient's address (instead of in record).

     -f     Read in the contents of the user's secondary mailbox MBOX (or the specified file) for
            processing; when S-nail is quit, it writes undeleted messages back to this file (but
            be aware of the hold option).  The optional file argument will undergo some special
            Filename transformations (as via file).  Note that file is not an argument to the
            flag -f, but is instead taken from the command line after option processing has been
            completed.  In order to use a file that starts with a hyphen-minus, prefix with a
            relative path, as in ‘./-hyphenbox.mbox’.

     -H     Display a summary of headers for the given file (depending on -u, inbox or MAIL, or
            as specified via -f).  A configurable summary view is available via the option -L.
            This mode does not honour showlast.

     -h     Show a short usage summary.

     -i     set ignore to ignore tty interrupt signals.

     -L spec
            Display a summary of headers of all messages that match the given spec in the file
            found by the same algorithm used by -H, then exit.  See the section Specifying
            messages for the format of spec.  This mode does not honour showlast.

            If the -e option has been given in addition no header summary is produced, but S-nail
            will instead indicate via its exit status whether spec matched any messages (‘0’) or
            not (‘1’); note that any verbose output is suppressed in this mode and must instead
            be enabled explicitly (e.g., by using the option -v).

     -M type
            Special send mode that will flag standard input with the MIME ‘Content-Type:’ set to
            the given type and use it as the main message body.  [v15 behaviour may differ] Using
            this option will bypass processing of message-inject-head and message-inject-tail.
            Also see -q, -m, -t.

     -m file
            Special send mode that will MIME classify the specified file and use it as the main
            message body.  [v15 behaviour may differ] Using this option will bypass processing of
            message-inject-head and message-inject-tail.  Also see -q, -M, -t.

     -N     inhibit the initial display of message headers when reading mail or editing a mailbox
            folder by calling unset for the internal variable header.

     -n     Standard flag that inhibits reading the system wide /etc/s-nail.rc upon startup.  The
            option -: allows more control over the startup sequence; also see Resource files.

     -q file
            Special send mode that will initialize the message body with the contents of the
            specified file, which may be standard input ‘-’ only in non-interactive context.
            Also see -M, -m, -t.

     -R     Any mailbox folder a.k.a. file opened will be in read-only mode.

     -r from-addr
            Whereas the source address that appears in the from header of a message (or in the
            sender header if the former contains multiple addresses) is honoured by the built-in
            SMTP transport, it is not used by a file-based mta (Mail-Transfer-Agent) for the RFC
            5321 reverse-path used for relaying and delegating a message to its destination(s),
            for delivery errors etc., but it instead uses the local identity of the initiating
            user.

            When this command line option is used the given from-addr will be assigned to the
            internal variable from, but in addition the command line option -f from-addr will be
            passed to a file-based mta whenever a message is sent.  Shall from-addr include a
            user name the address components will be separated and the name part will be passed
            to a file-based mta individually via -F name.

            If an empty string is passed as from-addr then the content of the variable from (or,
            if that contains multiple addresses, sender) will be evaluated and used for this
            purpose whenever the file-based mta is contacted.  By default, without -r that is,
            neither -f nor -F command line options are used when contacting a file-based MTA,
            unless this automatic deduction is enforced by seting the internal variable
            r-option-implicit.

            Remarks: many default installations and sites disallow overriding the local user
            identity like this unless either the MTA has been configured accordingly or the user
            is member of a group with special privileges.  Passing an invalid address will cause
            an error.

     -S var[=value]
            set (or, with a prefix string ‘no’, as documented in INTERNAL VARIABLES, unset)
            variable and optionally assign value, if supported; [v15 behaviour may differ] the
            entire expression is evaluated as if specified within dollar-single-quotes (see
            Shell-style argument quoting) if the internal variable v15-compat is set.  If the
            operation fails the program will exit if any of errexit or posix are set.  Settings
            established via -S cannot be changed from within Resource files or an account switch
            initiated by -A.  They will become mutable again before commands registered via -X
            are executed.

     -s subject
            Specify the subject of the message to be sent.  Newline (NL) and carriage-return (CR)
            bytes are invalid and will be normalized to space (SP) characters.

     -t     The message given (on standard input) is expected to contain, separated from the
            message body by an empty line, one or multiple message headers.  Headers can span
            multiple consecutive lines if follow lines start with any amount of whitespace.  A
            line starting with the number sign ‘#’ in the first column is ignored.  Message
            recipients can be given via the message headers ‘To:’, ‘Cc:’, ‘Bcc:’ or ‘Fcc:’, they
            will be added to any recipients specified on the command line, and are likewise
            subject to expandaddr validity checks.  If a message subject is specified via
            ‘Subject:’ then it will be used in favour of one given on the command line.

            More optional headers are ‘Reply-To:’ (possibly overriding reply-to), ‘Sender:’
            (sender), ‘From:’ (from and / or option -r).  ‘Message-ID:’, ‘In-Reply-To:’,
            ‘References:’ and ‘Mail-Followup-To:’, by default created automatically dependent on
            message context, will be used if specified (a special address massage will however
            still occur for the latter).  Any other custom header field (also see -C, customhdr
            and ~^) is passed through entirely unchanged, and in conjunction with the options -~
            or -# it is possible to embed COMMAND ESCAPES.  Also see -M, -m, -q.

     -u user
            Initially read the primary system mailbox of user, appropriate privileges presumed;
            effectively identical to ‘-f %user’.

     -V     Show S-nails version and exit.  The command version will also show the list of
            features: ‘$ s-nail -Xversion -Xx’.

     -v     setting the internal variable verbose enables display of some informational context
            messages.  Using it twice increases the level of verbosity.

     -X cmd
            Add the given (or multiple for a multiline argument) cmd to the list of commands to
            be executed, as a last step of program startup, before normal operation starts.  This
            is the only possibility to execute commands in non-interactive mode when reading
            startup files has been disabled.  The commands will be evaluated as a unit, just as
            via source.  Correlates with -# and errexit.

     -~     Enable COMMAND ESCAPES in compose mode even in non-interactive use cases.  This can
            be used to, e.g., automatically format the composed message text before sending the
            message:

                  $ ( echo 'line    one. Word.     Word2.';\
                      echo '~| /usr/bin/fmt -tuw66' ) |\
                    LC_ALL=C s-nail -d~:/ -Sttycharset=utf-8 bob@exam.ple

     -#     Enables batch mode: standard input is made line buffered, the complete set of
            (interactive) commands is available, processing of COMMAND ESCAPES is enabled in
            compose mode, and diverse INTERNAL VARIABLES are adjusted for batch necessities,
            exactly as if done via set: emptystart, noerrexit, noheader, noposix, quiet,
            sendwait, typescript-mode as well as MAIL, MBOX and inbox (the latter three to
            /dev/null).  The following prepares an email message in a batched dry run:

                  $ LC_ALL=C printf 'm bob\n~s ubject\nText\n~.\nx\n' |\
                    LC_ALL=C s-nail -d#:/ -X'alias bob bob@exam.ple'

     -.     This flag forces termination of option processing in order to prevent “option
            injection” (attacks).  It also forcefully puts S-nail into send mode, see On sending
            mail, and non-interactive mode.

     All given to-addr arguments and all receivers established via -b and -c are subject to the
     checks established by expandaddr, one of the INTERNAL VARIABLES; they all support the flag
     ‘shquote’.  If the setting of expandargv allows their recognition all mta-option arguments
     given at the end of the command line after a ‘--’ separator will be passed through to a
     file-based mta (Mail-Transfer-Agent) and persist for the entire session.  expandargv
     constraints do not apply to the content of mta-arguments.

   A starter
     S-nail is a direct descendant of BSD Mail, itself a successor to the Research UNIX mail
     which “was there from the start” according to HISTORY.  It thus represents the user side of
     the UNIX mail system, whereas the system side (Mail-Transfer-Agent, MTA) was traditionally
     taken by sendmail(8), and most MTAs provide a binary of this name for compatibility
     purposes.  If the [Option]al SMTP mta is included in the features of S-nail then the system
     side is not a mandatory precondition for mail delivery.

     Because S-nail strives for compliance with POSIX mailx(1) it is likely that some
     configuration settings have to be adjusted before using it is a smooth experience.  (Rather
     complete configuration examples can be found in the section EXAMPLES.)  The provided global
     /etc/s-nail.rc (one of the Resource files) template bends those standard imposed settings of
     the INTERNAL VARIABLES a bit towards more user friendliness and safety, however.

     For example, it sets hold and keepsave in order to suppress the automatic moving of messages
     to the secondary mailbox MBOX that would otherwise occur (see Message states), and keep to
     not remove empty system MBOX mailbox files (or all empty such files if posix
     a.k.a. POSIXLY_CORRECT mode has been enabled) to avoid mangling of file permissions when
     files eventually get recreated.

     It also enables sendwait in order to synchronize S-nail with the exit status report of the
     used mta when sending mails.  It sets emptystart to enter interactive startup even if the
     initial mailbox is empty, editheaders to allow editing of headers as well as fullnames to
     not strip down addresses in compose mode, and quote to include the message that is being
     responded to when replying, which is indented by an indentprefix that also deviates from
     standard imposed settings.  mime-counter-evidence is fully enabled, too.

     Some random remarks.  The file mode creation mask can be managed explicitly via the variable
     umask.  Sufficient system support provided symbolic links will not be followed when files
     are opened for writing.  Files and shell pipe output can be sourced for evaluation, also
     during startup from within the Resource files.

   On sending mail, and non-interactive mode
     To send a message to one or more people, using a local or built-in mta (Mail-Transfer-Agent)
     transport to actually deliver the generated mail message, S-nail can be invoked with
     arguments which are the names of people to whom the mail will be sent, and the command line
     options -b and -c can be used to add (blind) carbon copy receivers:

           # Via sendmail(1)
           $ s-nail -s ubject -a ttach.txt bill@exam.ple

           # But... try it in an isolated dry-run mode (-d) first
           $ LC_ALL=C s-nail -d -:/ -Ssendwait -Sttycharset=utf8 \
              -b bcc@exam.ple -c cc@exam.ple \
              -Sfullnames -. \
              '(Lovely) Bob <bob@exam.ple>' eric@exam.ple

           # With SMTP
           $ LC_ALL=C s-nail -d -:/ -Sv15-compat -Ssendwait -Sttycharset=utf8 \
               -S mta=smtps://mylogin@exam.ple:465 -Ssmtp-auth=none \
               -S from=scriptreply@exam.ple \
               -a /etc/mail.rc \
               -. eric@exam.ple < /tmp/letter.txt

     If standard input is a terminal rather than the message to be sent, the user is expected to
     type in the message contents.  In this compose mode S-nail treats lines beginning with the
     character ‘~’ special – these are so-called COMMAND ESCAPES, which can be used to read in
     files, process shell commands, add and edit attachments and more; e.g., ~v or ~e will start
     the VISUAL text EDITOR, respectively, to revise the message in its current state, ~h allows
     editing of the most important message headers, with the potent ~^ custom headers can be
     created, for example (more specifically than with -C and customhdr).  [Option]ally ~? gives
     an overview of most other available command escapes.

     The command escape ~. will leave compose mode and send the message once it is completed.
     Aborting letter composition is possible with either of ~x or ~q, the latter of which will
     save the message in the file denoted by DEAD unless nosave is set.  And unless ignoreeof is
     set the effect of ~. can also be achieved by typing end-of-transmission (EOT) via
     ‘control-D’ (‘^D’) at the beginning of an empty line, and ~q is always reachable by typing
     end-of-text (ETX) twice via ‘control-C’ (‘^C’).

     A number of ENVIRONMENT and INTERNAL VARIABLES can be used to alter default behavior.  E.g.,
     messages are sent asynchronously, without supervision, unless the internal variable sendwait
     is set, therefore send errors will not be recognizable until then.  setting (also via -S)
     editalong will automatically startup an editor when compose mode is entered, and editing of
     headers additionally to plain body content can be enabled via editheaders: [v15 behaviour
     may differ] some, but not all headers can be created, edited or deleted in an editor, then.
     askcc and askbcc will cause the user to be prompted actively for (blind) carbon-copy
     recipients, respectively, and (the default) asksend will request confirmation whether the
     message shall be sent.

     The envelope sender address is defined by from, explicitly defining an originating hostname
     may be desirable, especially with the built-in SMTP Mail-Transfer-Agent mta.  Character sets
     for outgoing message and MIME part content are configurable via sendcharsets, whereas input
     data is assumed to be in ttycharset.  Message data will be passed over the wire in a
     mime-encoding.  MIME parts a.k.a. attachments need to be assigned a mimetype, usually taken
     out of The mime.types files.  Saving a copy of sent messages in a record mailbox may be
     desirable – as for most mailbox file targets the value will undergo Filename
     transformations.  Some introductional -d or debug sandbox dry-run tests will prove
     correctness.

     Message recipients (as specified on the command line or defined in ‘To:’, ‘Cc:’ or ‘Bcc:’)
     are subject to alternates filtering, and may not only be email addressees but can also be
     names of mailboxes and even complete shell command pipe specifications.  If the variable
     expandaddr is not set then only network addresses (see mailaddr(7) for a description of mail
     addresses) and plain user names (including MTA aliases) may be used, other types will be
     filtered out, giving a warning message.  A network address that contains no domain-, but
     only a valid local user ‘<name>’ in angle brackets will be automatically expanded to a valid
     address when hostname is set to a non-empty value; setting it to the empty value instructs
     S-nail that the used mta will perform the necessary expansion.  The command addrcodec may
     help to generate standard compliant network addresses.

     If the variable expandaddr is set then an extended set of recipient addresses will be
     accepted: Any name that starts with a vertical bar ‘|’ character specifies a command pipe –
     the command string following the ‘|’ is executed and the message is sent to its standard
     input; Likewise, any name that consists only of hyphen-minus ‘-’ or starts with the
     character solidus ‘/’ or the character sequence dot solidus ‘./’ is treated as a file,
     regardless of the remaining content.  Any other name which contains a commercial at ‘@’
     character is a network address; Any other name which starts with a plus sign ‘+’ character
     is a mailbox name; Any other name which contains a solidus ‘/’ character but no exclamation
     mark ‘!’ or percent sign ‘%’ character before is also a mailbox name; What remains is
     treated as a network address.

           $ echo bla | s-nail -Sexpandaddr -s test ./mbox.mbox
           $ echo bla | s-nail -Sexpandaddr -s test '|cat >> ./mbox.mbox'
           $ echo safe | LC_ALL=C \
               s-nail -:/ -Sv15-compat -Ssendwait -Sttycharset=utf8 \
                 -Sexpandaddr=fail,-all,+addr,failinvaddr -s test \
                 -. bob@exam.ple

     To create file-carbon-copies the special recipient header ‘Fcc:’ may be used as often as
     desired.  Its entire value (or body in standard terms) is interpreted as a file target,
     after having been subject to Filename transformations.  Beside using the command escape ~^
     (to create a ‘Fcc’ header) this is the only way to create a file-carbon-copy without
     introducing an ambiguity regarding the interpretation of the address, e.g., to use file
     names with leading vertical bars or commercial ats.  Like all other recipients ‘Fcc:’ is
     subject to the checks of expandaddr.

     It is possible to create personal distribution lists via the alias command, so that, for
     instance, the user can send mail to ‘cohorts’ and have it go to a group of people.
     Different to the alias mechanism of a local mta, which is often tracked in a file
     /etc/aliases, documented in aliases(5), and the names of which are subject to the ‘name’
     constraint of expandaddr, personal aliases will be expanded by S-nail before the message is
     sent.  They are thus a convenient alternative to specifying each addressee by itself,
     correlate with the active set of alternates, and are subject to metoo filtering.

           ? alias  cohorts  bill jkf mark kridle@ucbcory ~/cohorts.mbox
           ? alias  mark  mark@exam.ple

     on-compose-enter, on-compose-leave and on-compose-cleanup hook variables may be set to
     defined macros to automatically adjust some settings dependent on receiver, sender or
     subject contexts, and via the on-compose-splice as well as on-compose-splice-shell
     variables, the former also to be set to a defined macro, increasingly powerful mechanisms to
     perform automated message adjustments, including signature creation, are available.  ([v15
     behaviour may differ] These hooks work for commands which newly create messages, namely
     forward, mail, reply and variants; resend and Resend for now provide only the hooks
     on-resend-enter and on-resend-cleanup.)

     For the purpose of arranging a complete environment of settings that can be switched to with
     a single command or command line option there are accounts.  Alternatively it is also
     possible to use a flat configuration, making use of so-called variable chains which
     automatically pick ‘USER@HOST’ or ‘HOST’ context-dependent variable variants: for example
     addressing ‘File pop3://yaa@exam.ple’ would find pop3-no-apop-yaa@exam.ple,
     pop3-no-apop-exam.ple and pop3-no-apop in order.  See On URL syntax and credential lookup
     and INTERNAL VARIABLES.

     To avoid environmental noise scripts should “detach” S-nail from any configuration files and
     create a script-local environment, ideally with the command line options -: to disable any
     configuration file in conjunction with repetitions of -S to specify variables:

           $ env LC_ALL=C s-nail -:/ \
               -Sv15-compat -Ssendwait -Sttycharset=utf-8 \
               -Sexpandaddr=fail,-all,failinvaddr \
               -S mta=smtps://mylogin@exam.ple:465 -Ssmtp-auth=login \
               -S from=scriptreply@exam.ple \
               -s 'Subject to go' -a attachment_file \
               -Sfullnames -. \
               'Recipient 1 <rec1@exam.ple>' rec2@exam.ple \
               < content_file

     As shown, scripts can “fake” a locale environment, the above specifies the all-compatible
     7-bit clean LC_ALL “C”, but will nonetheless take and send UTF-8 in the message text by
     using ttycharset.  In interactive mode, which is introduced in the next section, messages
     can be sent by calling the mail command with a list of recipient addresses:

           $ s-nail -d -Squiet -Semptystart
           "/var/spool/mail/user": 0 messages
           ? mail "Recipient 1 <rec1@exam.ple>", rec2@exam.ple
           ...
           ? # Will do the right thing (tm)
           ? m rec1@exam.ple rec2@exam.ple

   On reading mail, and interactive mode
     When invoked without addressees S-nail enters interactive mode in which mails may be read.
     When used like that the user's system inbox (for more on mailbox types please see the
     command file) is read in and a one line header of each message therein is displayed if the
     variable header is set.  The visual style of this summary of headers can be adjusted through
     the variable headline and the possible sorting criterion via autosort.  Scrolling through
     screenfuls of headers can be performed with the command z.  If the initially opened mailbox
     is empty S-nail will instead exit immediately (after displaying a message) unless the
     variable emptystart is set.

     At the prompt the command list will give a listing of all available commands and help will
     [Option]ally give a summary of some common ones.  If the [Option]al documentation strings
     are available (see features) one can type ‘help X’ (or ‘?X’) and see the actual expansion of
     ‘X’ and what its purpose is, i.e., commands can be abbreviated (note that POSIX defines some
     abbreviations, so that the alphabetical order of commands does not necessarily relate to the
     abbreviations; it is however possible to define overwrites with commandalias).  These
     commands can also produce a more verbose output.

     Messages are given numbers (starting at 1) which uniquely identify messages; the current
     message – the “dot” – will either be the first new message, or the first unread message, or
     the first message of the mailbox; the internal variable showlast will instead cause usage of
     the last message for this purpose.  The command headers will display a screenful of header
     summaries containing the “dot”, whereas from will display only the summaries of the given
     messages, defaulting to the “dot”.

     Message content can be displayed with the command type (‘t’, alias print).  Here the
     variable crt controls whether and when S-nail will use the configured PAGER for display
     instead of directly writing to the user terminal screen, the sole difference to the command
     more, which will always use the PAGER.  The command top will instead only show the first
     toplines of a message (maybe even compressed if topsqueeze is set).  Message display
     experience may improve by setting and adjusting mime-counter-evidence, and also see HTML
     mail and MIME attachments.

     By default the current message (“dot”) is displayed, but like with many other commands it is
     possible to give a fancy message specification (see Specifying messages), e.g., ‘t:u’ will
     display all unread messages, ‘t.’ will display the “dot”, ‘t 1 5’ will type the messages 1
     and 5, ‘t 1-5’ will type the messages 1 through 5, and ‘t-’ and ‘t+’ will display the
     previous and the next message, respectively.  The command search (a more substantial alias
     for from) will display a header summary of the given message specification list instead of
     their content, e.g., the following will search for subjects:

           ? from '@Some subject to search for'

     In the default setup all header fields of a message will be typed, but fields can be white-
     or blacklisted for a variety of applications by using the command headerpick, e.g., to
     restrict their display to a very restricted set for type: ‘headerpick type retain from to cc
     subject’.  In order to display all header fields of a message regardless of currently active
     ignore or retain lists, use the commands Type and Top; Show will show the raw message
     content.  Note that historically the global /etc/s-nail.rc not only adjusts the list of
     displayed headers, but also sets crt.  ([v15 behaviour may differ] A yet somewhat
     restricted) Reliable scriptable message inspection is available via digmsg.

     Dependent upon the configuration a line editor (see the section On terminal control and line
     editor) aims at making the user experience with the many COMMANDS a bit nicer.  When reading
     the system inbox, or when -f (or file) specified a mailbox explicitly prefixed with the
     special ‘%:’ modifier (to propagate it to a primary system mailbox), then messages which
     have been read (see Message states) will be automatically moved to a secondary mailbox, the
     user's MBOX file, when the mailbox is left, either by changing the active mailbox or by
     quitting S-nail – this automatic moving from a system- or primary- to the secondary mailbox
     is not performed when the variable hold is set.  Messages can also be explicitly moved to
     other mailboxes, whereas copy keeps the original message.  write can be used to write out
     data content of specific parts of messages.

     After examining a message the user can reply ‘r’ to the sender and all recipients (which
     will also be placed in ‘To:’ unless recipients-in-cc is set), or Reply ‘R’ exclusively to
     the sender(s).  The command Lreply knows how to apply a special addressee massage, see
     Mailing lists.  Dependent on the presence and value of quote the message being replied to
     will be included in a quoted form.  forwarding a message will allow editing the new message:
     the original message will be contained in the message body, adjusted according to
     headerpick.  It is possible to resend or Resend messages: the former will add a series of
     ‘Resent-’ headers, whereas the latter will not; different to newly created messages editing
     is not possible and no copy will be saved even with record unless the additional variable
     record-resent is set.  When sending, replying or forwarding messages comments and full names
     will be stripped from recipient addresses unless the internal variable fullnames is set.

     Of course messages can be delete ‘d’, and they can spring into existence again via undelete,
     or when the S-nail session is ended via the exit or xit commands to perform a quick program
     termation.  To end a mail processing session regulary and perform a full program exit one
     may issue the command quit.  It will, among others, move read messages to the secondary
     mailbox MBOX as necessary, discard deleted messages in the current mailbox, and update the
     [Option]al (see features) line editor history-file.

   HTML mail and MIME attachments
     Messages which are HTML-only become more and more common, and of course many messages come
     bundled with a bouquet of MIME (Multipurpose Internet Mail Extensions) parts.  To get a
     notion of MIME types S-nail has a default set of types built-in, onto which the content of
     The mime.types files will be added (as configured and allowed by mimetypes-load-control).
     Types can also become registered with the command mimetype.  To improve interaction with
     faulty MIME part declarations which are often seen in real-life messages, setting
     mime-counter-evidence will allow verification of the given assertion, and possible provision
     of an alternative, better MIME type.

     Whereas S-nail [Option]ally supports a simple HTML-to-text filter for displaying HTML
     messages, it cannot handle MIME types other than plain text itself.  Instead programs need
     to become registered to deal with specific MIME types or file extensions.  These programs
     may either prepare plain text versions of their input in order to enable S-nail to integrate
     their output neatlessly in its own message visualization (a mode which is called
     copiousoutput), or display the content themselves, for example in an external graphical
     window: such handlers will only be considered by and for the command mimeview.

     To install a handler program for a specific MIME type an according pipe-TYPE/SUBTYPE
     variable needs to be set; to instead define a handler for a specific file extension the
     respective pipe-EXTENSION variable can be used – these handlers take precedence.
     [Option]ally S-nail supports mail user agent configuration as defined in RFC 1524; this
     mechanism (see The Mailcap files) will be queried for display or quote handlers if none of
     the former two did; it will be the sole source for handlers of other purpose.  A last source
     for handlers is the MIME type definition itself, if a type-marker has been registered with
     the command mimetype, which many of the built-in MIME types do.

     For example, to display a HTML message inline (converted to a more fancy plain text
     representation than the built-in filter is capable to produce) with either of the text-mode
     browsers lynx(1) or elinks(1), teach S-nail about MathML documents and make it display them
     as plain text, and to open PDF attachments in an external PDF viewer, asynchronously and
     with some other magic attached:

           ? if [ "$features" !% +filter-html-tagsoup ]
           ?   #set pipe-text/html='@* elinks -force-html -dump 1'
           ?   set pipe-text/html='@* lynx -stdin -dump -force_html'
           ?   # Display HTML as plain text instead
           ?   #set pipe-text/html=@
           ? endif
           ? mimetype @ application/mathml+xml mathml
           ? wysh set pipe-application/pdf='@&=@ \
               trap "rm -f \"${MAILX_FILENAME_TEMPORARY}\"" EXIT;\
               trap "trap \"\" INT QUIT TERM; exit 1" INT QUIT TERM;\
               mupdf "${MAILX_FILENAME_TEMPORARY}"'

   Mailing lists
     S-nail offers some support to ease handling of mailing lists.  The command mlist promotes
     all given arguments to known mailing lists, and mlsubscribe sets their subscription
     attribute, creating them first as necessary.  (On the other hand unmlsubscribe does not
     unmlist automatically, but only resets the subscription attribute.)  Using the commands
     without arguments will show (a subset of) all currently defined mailing lists.  The headline
     format ‘%T’ can be used to mark out messages with configured list addresses in the display
     of headers.

     If the [Option]al regular expression support is available a mailing list specification that
     contains any of the “magical” regular expression characters ‘^[]*+?|$’ (see re_format(7))
     will be interpreted as one, which allows matching of many addresses with a single
     expression.  However, all fully qualified list addresses are matched via a fast dictionary,
     whereas expressions are placed in (a) list(s) which is (are) matched sequentially.

           ? set followup-to followup-to-honour=ask-yes \
               reply-to-honour=ask-yes
           ? wysh mlist a1@b1.c1 a2@b2.c2 '.*@lists\.c3$'
           ? mlsubscribe a4@b4.c4 exact@lists.c3

     The variable followup-to-honour will ensure that a ‘Mail-Followup-To:’ header is honoured
     when the message is being replied to (via reply and Lreply) and followup-to controls whether
     this header is created when sending mails; it will be created automatically for a couple of
     reasons, too, like when the special “mailing list specific” respond command Lreply is used,
     when reply is used to respond to a message with its ‘Mail-Followup-To:’ being honoured etc.

     A difference in between the handling of known and subscribed lists is that the address of
     the user is usually not part of a generated ‘Mail-Followup-To:’ when addressing the latter,
     whereas it is for the former kind of lists.  Usually because there are exceptions: say, if
     multiple lists are addressed and not all of them are subscribed lists.

     For convenience S-nail will, temporarily, automatically add a list address that is presented
     in the ‘List-Post:’ header of a message that is being responded to to the list of known
     mailing lists.  Shall that header have existed S-nail will instead, dependent on the
     variable reply-to-honour, use an also set ‘Reply-To:’ for this purpose (if it provides a
     single address which resides on the same domain as what is stated in ‘List-Post:’) in order
     to accept a list administrator's wish that is supposed to have been manifested like that.

   Signed and encrypted messages with S/MIME
     [Option] S/MIME provides two central mechanisms: message signing and message encryption.  A
     signed message contains some data in addition to the regular text.  The data can be used to
     verify that the message has been sent using a valid certificate, that the sender address
     matches that in the certificate, and that the message text has not been altered.  Signing a
     message does not change its regular text; it can be read regardless of whether the
     recipients software is able to handle S/MIME.  It is thus usually possible to sign all
     outgoing messages if so desired.

     Encryption, in contrast, makes the message text invisible for all people except those who
     have access to the secret decryption key.  To encrypt a message, the specific recipients
     public encryption key must be known.  It is therefore not possible to send encrypted mail to
     people unless their key has been retrieved from either previous communication or public key
     directories.  Because signing is performed with private keys, and encryption with public
     keys, messages should always be signed before becoming encrypted.

     A central concept to S/MIME is that of the certification authority (CA).  A CA is a trusted
     institution that issues certificates.  For each of these certificates it can be verified
     that it really originates from the CA, provided that the CA's own certificate is previously
     known.  A set of CA certificates is usually delivered and installed together with the
     cryptographical library that is used on the local system.  Therefore reasonable security for
     S/MIME on the Internet is provided if the source that provides that library installation is
     trusted.  It is also possible to use a specific pool of trusted certificates.  If this is
     desired, smime-ca-no-defaults should be set to avoid using the default certificate pool, and
     smime-ca-file and/or smime-ca-dir should be pointed to a trusted pool of certificates.  A
     certificate cannot be more secure than the method its CA certificate has been retrieved
     with.

     This trusted pool of certificates is used by the command verify to ensure that the given
     S/MIME messages can be trusted.  If so, verified sender certificates that were embedded in
     signed messages can be saved locally with the command certsave, and used by S-nail to
     encrypt further communication with these senders:

           ? certsave FILENAME
           ? set smime-encrypt-USER@HOST=FILENAME \
               smime-cipher-USER@HOST=AES256

     To sign outgoing messages, in order to allow receivers to verify the origin of these
     messages, a personal S/MIME certificate is required.  S-nail supports password-protected
     personal certificates (and keys), see smime-sign-cert.  The section On URL syntax and
     credential lookup gives an overview of the possible sources of user credentials, and S/MIME
     step by step shows examplarily how a private S/MIME certificate can be obtained.  In
     general, if such a private key plus certificate “pair” is available, all that needs to be
     done is to set some variables:

           ? set smime-sign-cert=ME@exam.ple.paired \
               smime-sign-digest=SHA512 \
               smime-sign

     Variables of interest for S/MIME in general are smime-ca-dir, smime-ca-file, smime-ca-flags,
     smime-ca-no-defaults, smime-crl-dir, smime-crl-file.  For S/MIME signing of interest are
     smime-sign, smime-sign-cert, smime-sign-include-certs and smime-sign-digest.  Additional
     variables of interest for S/MIME en- and decryption: smime-cipher and
     smime-encrypt-USER@HOST.

     [v15 behaviour may differ] Note that neither S/MIME signing nor encryption applies to
     message subjects or other header fields yet.  Thus they may not contain sensitive
     information for encrypted messages, and cannot be trusted even if the message content has
     been verified.  When sending signed messages, it is recommended to repeat any important
     header information in the message text.

   On URL syntax and credential lookup
     [v15-compat] For accessing protocol-specific resources usage of Uniform Resource Locators
     (URL, RFC 1738) has become omnipresent.  S-nail expects and understands URLs in the
     following form; parts in brackets ‘[]’ denote optional parts, optional either because there
     also exist other ways to define the information in question or because support of the part
     is protocol-specific, e.g., ‘/path’ is used by the [Option]al Maildir directory and the IMAP
     protocol, but not by POP3; If any of ‘USER’ and ‘PASSWORD’ are specified they must be given
     in URL percent encoded form (RFC 3986; the command urlcodec may be helpful):

           PROTOCOL://[USER[:PASSWORD]@]server[:port][/path]

     Note that these S-nail URLs most often do not conform to any real standard, but instead
     represent a normalized variant of RFC 1738 – they are not used in data exchange but only
     meant as a compact, easy-to-use way of defining and representing information in a well-known
     notation.

     Many internal variables of S-nail exist in multiple versions, called variable chains for the
     rest of this document: the plain ‘variable’ as well as ‘variable-HOST’ and
     ‘variable-USER@HOST’.  Here ‘HOST’ indeed means ‘server:port’ if a ‘port’ had been specified
     in the respective URL, otherwise it refers to the plain ‘server’.  Also, ‘USER’ is not truly
     the ‘USER’ that had been found when doing the user chain lookup as is described below, i.e.,
     this ‘USER’ will never be in URL percent encoded form, whether it came from an URL or not;
     i.e., variable chain name extensions of INTERNAL VARIABLES must not be URL percent encoded.

     For example, whether an hypothetical URL ‘smtp://hey%3Ayou@our.house’ had been given that
     includes a user, or whether the URL was ‘smtp://our.house’ and the user had been found
     differently, to lookup the variable chain smtp-use-starttls S-nail first looks for whether
     ‘smtp-use-starttls-hey:you@our.house’ is defined, then whether ‘smtp-use-starttls-our.house’
     exists before finally ending up looking at the plain variable itself.

     S-nail obeys the following logic scheme when dealing with the necessary credential
     information of an account:

     ·   If no ‘USER’ has been given in the URL the variables user-HOST and user are looked up.
         If no such variable(s) can be found then S-nail will, when enforced by the [Option]al
         variables netrc-lookup-HOST or netrc-lookup, search The .netrc file of the user for a
         ‘HOST’ specific entry which provides a ‘login’ name: this lookup will only succeed if
         unambiguous (one possible matching entry for ‘HOST’).

         If there is still no ‘USER’ then S-nail will fall back to the user who is supposed to
         run S-nail, the identity of which has been fixated during S-nail startup and is known to
         be a valid user on the current host.

     ·   Authentication: unless otherwise noted this will lookup the PROTOCOL-auth-USER@HOST,
         PROTOCOL-auth-HOST, PROTOCOL-auth variable chain, falling back to a protocol-specific
         default should this have no success.

     ·   If no ‘PASSWORD’ has been given in the URL, then if the ‘USER’ has been found through
         the [Option]al netrc-lookup that may have already provided the password, too.  Otherwise
         the variable chain password-USER@HOST, password-HOST, password is looked up and used if
         existent.

         Afterwards the complete [Option]al variable chain netrc-lookup-USER@HOST,
         netrc-lookup-HOST, netrc-lookup is looked up.  If set, the netrc cache is searched for a
         password only (multiple user accounts for a single machine may exist as well as a
         fallback entry without user but with a password).

         If at that point there is still no password available, but the (protocols') chosen
         authentication type requires a password, then in interactive mode the user will be
         prompted on the terminal.

     Note: S/MIME verification works relative to the values found in the ‘From:’ (or ‘Sender:’)
     header field(s), which means that the values of smime-sign, smime-sign-cert,
     smime-sign-include-certs and smime-sign-digest will not be looked up using the ‘USER’ and
     ‘HOST’ chains from above but instead use the corresponding values from the message that is
     being worked on.  In unusual cases multiple and different ‘USER’ and ‘HOST’ combinations may
     therefore be involved – on the other hand those unusual cases become possible.  The usual
     case is as short as:

           set mta=smtp://USER:PASS@HOST smtp-use-starttls \
               smime-sign smime-sign-cert=+smime.pair

     The section EXAMPLES contains complete example configurations.

   Encrypted network communication
     SSL (Secure Sockets Layer) a.k.a. its successor TLS (Transport Layer Security) are protocols
     which aid in securing communication by providing a safely initiated and encrypted network
     connection.  A central concept of TLS is that of certificates: as part of each network
     connection setup a (set of) certificates will be exchanged, and by using those the identity
     of the network peer can be cryptographically verified; if possible the TLS/SNI
     (ServerNameIndication) extension will be enabled in order to allow servers fine-grained
     control over the certificates being used.  TLS works by using a locally installed pool of
     trusted certificates, and verifying the connection peer succeeds if that provides a
     certificate which has been issued or is trusted by any certificate in the trusted local
     pool.

     The local pool of trusted so-called CA (Certification Authority) certificates is usually
     delivered with the used TLS library, and will be selected automatically.  It is also
     possible to use a specific pool of trusted certificates.  If this is desired,
     tls-ca-no-defaults should be set to avoid using the default certificate pool, and
     tls-ca-file and/or (with special preparation) tls-ca-dir should be pointed to a trusted pool
     of certificates.  A certificate cannot be more secure than the method its CA certificate has
     been retrieved with.  For inspection or other purposes, the certificate of a server (as seen
     when connecting to it) can be fetched like this:

           $ </dev/null openssl s_client -showcerts -connect \
               the-server.example:pop3s 2>&1 | tee log.txt

     S-nail also supports a mode of operation in which certificates are not at all matched
     against a local pool of CA certificates.  Instead a message digest will be calculated for
     the certificate presented by the connection peer, and be compared against tls-fingerprint (a
     variable chain that picks up ‘USER@HOST’ or ‘HOST’ context-dependent variable variants), and
     the connection will succeed if the calculated digest equals the expected one.  The used
     message digest can be configured via (the chain) tls-fingerprint-digest.  The command tls
     may be helpful.

     It depends on the used protocol whether encrypted communication is possible, and which
     configuration steps have to be taken to enable it.  Some protocols, e.g., POP3S, are
     implicitly encrypted, others, like POP3, can upgrade a plain text connection if so
     requested.  For example, to use the ‘STLS’ that POP3 offers (a member of) the variable
     (chain) pop3-use-starttls needs to be set:

           shortcut encpop1 pop3s://pop1.exam.ple

           shortcut encpop2 pop3://pop2.exam.ple
           set pop3-use-starttls-pop2.exam.ple

           set mta=smtps://smtp.exam.ple:465
           set mta=smtp://smtp.exam.ple smtp-use-starttls

     Normally that is all there is to do, given that TLS libraries try to provide safe defaults,
     plenty of knobs however exist to adjust settings.  For example certificate verification
     settings can be fine-tuned via tls-ca-flags, and the TLS configuration basics are accessible
     via tls-config-pairs, for example to specify the allowed protocols or cipher lists that a
     communication channel may use.  In the past hints on how to restrict the set of protocols to
     highly secure ones were indicated, but as of the time of this writing the list of protocols
     or ciphers may need to become relaxed in order to be able to connect to some servers; the
     following example allows connecting to a “Lion” that uses OpenSSL 0.9.8za from June 2014
     (refer to INTERNAL VARIABLES for more on variable chains):

           wysh set tls-config-pairs-lion@exam.ple='MinProtocol=TLSv1.1,\
               CipherString=TLSv1.2:!aNULL:!eNULL:\
                 ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:\
                 DHE-RSA-AES256-SHA:@STRENGTH'

     The OpenSSL program ciphers(1) can be used and should be referred to when creating a custom
     cipher list.  Variables of interest for TLS in general are tls-ca-dir, tls-ca-file,
     tls-ca-flags, tls-ca-no-defaults, tls-config-file, tls-config-module, tls-config-pairs,
     tls-crl-dir, tls-crl-file, tls-rand-file as well as tls-verify.

   Character sets
     [Option] S-nail detects the character set of the terminal by using mechanisms that are
     controlled by the LC_CTYPE environment variable (in fact LC_ALL, LC_CTYPE, LANG, in that
     order, see there).  The internal variable ttycharset will be set to the detected terminal
     character set accordingly, and will thus show up in the output of commands like, e.g., set
     and varshow.

     However, the user may give ttycharset a value during startup, making it possible to send
     mail in a completely “faked” locale environment, an option which can be used to generate and
     send, e.g., 8-bit UTF-8 input data in a pure 7-bit US-ASCII ‘LC_ALL=C’ environment (an
     example of this can be found in the section On sending mail, and non-interactive mode).
     Changing the value does not mean much beside that, because several aspects of the real
     character set are implied by the locale environment of the system, which stays unaffected by
     ttycharset.

     Messages and attachments which consist of 7-bit clean data will be classified as consisting
     of charset-7bit character data.  This is a problem if the ttycharset character set is a
     multibyte character set that is also 7-bit clean.  For example, the Japanese character set
     ISO-2022-JP is 7-bit clean but capable to encode the rich set of Japanese Kanji, Hiragana
     and Katakana characters: in order to notify receivers of this character set the mail message
     must be MIME encoded so that the character set ISO-2022-JP can be advertised!  To achieve
     this, the variable charset-7bit must be set to ISO-2022-JP.  (Today a better approach
     regarding email is the usage of UTF-8, which uses 8-bit bytes for non-US-ASCII data.)

     If the [Option]al character set conversion capabilities are not available (features does not
     include the term ‘+iconv’), then ttycharset will be the only supported character set, it is
     simply assumed that it can be used to exchange 8-bit messages (over the wire an
     intermediate, configurable mime-encoding may be applied), and the rest of this section does
     not apply; it may however still be necessary to explicitly set it if automatic detection
     fails, since in that case it defaults to LATIN1 a.k.a. ISO-8859-1 unless the operating
     system environment is known to always and exclusively support UTF-8 locales.

     [Option] When reading messages, their text is converted into ttycharset as necessary in
     order to display them on the user's terminal.  Unprintable characters and invalid byte
     sequences are detected and replaced by proper substitution characters.  Character set
     mappings for source character sets can be established with the command charsetalias, which
     may be handy to work around faulty character set catalogues (e.g., to add a missing LATIN1
     to ISO-8859-1 mapping), or to enforce treatment of one character set as another one (e.g.,
     to interpret LATIN1 as CP1252).  Also see charset-unknown-8bit to deal with another hairy
     aspect of message interpretation.

     When sending messages their parts and attachments are classified.  Whereas no character set
     conversion is performed on those parts which appear to be binary data, the character set
     being used must be declared within the MIME header of an outgoing text part if it contains
     characters that do not conform to the set of characters that are allowed by the email
     standards.  Permissible values for character sets used in outgoing messages can be declared
     using the sendcharsets variable, and charset-8bit, which defines a catch-all last-resort
     fallback character set that is implicitly appended to the list of character sets in
     sendcharsets.

     When replying to a message and the variable reply-in-same-charset is set, then the character
     set of the message being replied to is tried first (still being a subject of charsetalias).
     And it is also possible to make S-nail work even more closely related to the current locale
     setting automatically by using the variable sendcharsets-else-ttycharset, please see there
     for more information.

     All the specified character sets are tried in order unless the conversion of the part or
     attachment succeeds.  If none of the tried (8-bit) character sets is capable to represent
     the content of the part or attachment, then the message will not be send and its text will
     optionally be saved in DEAD.  In general, if a message saying “cannot convert from a to b”
     appears, either some characters are not appropriate for the currently selected (terminal)
     character set, or the needed conversion is not supported by the system.  In the first case,
     it is necessary to set an appropriate LC_CTYPE locale and/or the variable ttycharset.

     The best results are usually achieved when S-nail is run in a UTF-8 locale on an UTF-8
     capable terminal, in which case the full Unicode spectrum of characters is available.  In
     this setup characters from various countries can be displayed, while it is still possible to
     use more simple character sets for sending to retain maximum compatibility with older mail
     clients.

     On the other hand the POSIX standard defines a locale-independent 7-bit “portable character
     set” that should be used when overall portability is an issue, the even more restricted
     subset named “portable filename character set” consists of A-Z, a-z, 0-9, period ‘.’,
     underscore ‘_’ and hyphen-minus ‘-’.

   Message states
     S-nail differentiates in between several message states; the current state will be reflected
     in the summary of headers if the attrlist of the configured headline allows, and Specifying
     messages dependent on their state is possible.  When operating on the system inbox, or in
     any other primary system mailbox, special actions, like the automatic moving of messages to
     the secondary mailbox MBOX, may be applied when the mailbox is left (also implicitly by
     program termination, unless the command exit was used) – however, because this may be
     irritating to users which are used to “more modern” mail-user-agents, the provided global /
     etc/s-nail.rc template sets the internal hold and keepsave variables in order to suppress
     this behaviour.

     ‘new’     Message has neither been viewed nor moved to any other state.  Such messages are
               retained even in the primary system mailbox.

     ‘unread’  Message has neither been viewed nor moved to any other state, but the message was
               present already when the mailbox has been opened last: Such messages are retained
               even in the primary system mailbox.

     ‘read’    The message has been processed by one of the following commands: ~f, ~m, ~F, ~M,
               copy, mbox, next, pipe, Print, print, top, Type, type, undelete.  The commands dp
               and dt will always try to automatically “step” and type the “next” logical
               message, and may thus mark multiple messages as read, the delete command will do
               so if the internal variable autoprint is set.

               Except when the exit command is used, messages that are in a primary system
               mailbox and are in ‘read’ state when the mailbox is left will be saved in the
               secondary mailbox MBOX unless the internal variable hold it set.

     ‘deleted’ The message has been processed by one of the following commands: delete, dp, dt.
               Only undelete can be used to access such messages.

     ‘preserved’ The message has been processed by a preserve command and it will be retained in
               its current location.

     ‘saved’   The message has been processed by one of the following commands: save or write.
               Unless when the exit command is used, messages that are in a primary system
               mailbox and are in ‘saved’ state when the mailbox is left will be deleted; they
               will be saved in the secondary mailbox MBOX when the internal variable keepsave is
               set.

     In addition to these message states, flags which otherwise have no technical meaning in the
     mail system except allowing special ways of addressing them when Specifying messages can be
     set on messages.  These flags are saved with messages and are thus persistent, and are
     portable between a set of widely used MUAs.

     answered  Mark messages as having been answered.

     draft     Mark messages as being a draft.

     flag      Mark messages which need special attention.

   Specifying messages
     [Only new quoting rules] Commands which take Message list arguments, such as from a.k.a.
     search, type and delete, can be given a list of message numbers as arguments to apply to a
     number of messages at once.  Thus ‘delete 1 2’ deletes messages 1 and 2, whereas ‘delete
     1-5’ will delete the messages 1 through 5.  In sorted or threaded mode (see the sort
     command), ‘delete 1-5’ will delete the messages that are located between (and including)
     messages 1 through 5 in the sorted/threaded order, as shown in the headers summary.  The
     following special message names exist:

     .     The current message, the so-called “dot”.

     ;     The message that was previously the current message; needs to be quoted.

     ,     The parent message of the current message, that is the message with the Message-ID
           given in the ‘In-Reply-To:’ field or the last entry of the ‘References:’ field of the
           current message.

     -     The previous undeleted message, or the previous deleted message for the undelete
           command; In sorted or ‘thread’ed mode, the previous such message in the according
           order.

     +     The next undeleted message, or the next deleted message for the undelete command; In
           sorted or ‘thread’ed mode, the next such message in the according order.

     ^     The first undeleted message, or the first deleted message for the undelete command; In
           sorted or ‘thread’ed mode, the first such message in the according order.

     $     The last message; In sorted or ‘thread’ed mode, the last such message in the according
           order.  Needs to be quoted.

     &x    In ‘thread’ed sort mode, selects the message addressed with x, where x is any other
           message specification, and all messages from the thread that begins at it.  Otherwise
           it is identical to x.  If x is omitted, the thread beginning with the current message
           is selected.

     *     All messages.

     `     All messages that were included in the Message list arguments of the previous command;
           needs to be quoted.

     x-y   An inclusive range of message numbers.  Selectors that may also be used as endpoints
           include any of .;-+^$.

     address
           A case-insensitive “any substring matches” search against the ‘From:’ header, which
           will match addresses (too) even if showname is set (and POSIX says “any address as
           shown in a header summary shall be matchable in this form”); However, if the allnet
           variable is set, only the local part of the address is evaluated for the comparison,
           not ignoring case, and the setting of showname is completely ignored.  For finer
           control and match boundaries use the ‘@’ search expression.

     /string
           All messages that contain string in the subject field (case ignored according to
           locale).  See also the searchheaders variable.  If string is empty, the string from
           the previous specification of that type is used again.

     [@name-list]@expr
           All messages that contain the given case-insensitive search expression;  If the
           [Option]al regular expression support is available expr will be interpreted as (an
           extended) one if any of the “magical” regular expression characters ‘^[]*+?|$’ is seen
           (see re_format(7)).  If the optional @name-list part is missing the search is
           restricted to the subject field body, but otherwise name-list specifies a comma-
           separated list of header fields to search, e.g.,

                 '@to,from,cc@Someone i ought to know'

           In order to search for a string that includes a ‘@’ (commercial at) character the
           name-list is effectively non-optional, but may be given as the empty string.  Also,
           specifying an empty search expression will effectively test for existence of the given
           header fields.  Some special header fields may be abbreviated: ‘f’, ‘t’, ‘c’, ‘b’ and
           ‘s’ will match ‘From’, ‘To’, ‘Cc’, ‘Bcc’ and ‘Subject’, respectively and case-
           insensitively.  [Option]ally, and just like expr, name-list will be interpreted as (an
           extended) regular expression if any of the “magical” regular expression characters is
           seen.

           The special names ‘header’ or ‘<’ can be used to search in (all of) the header(s) of
           the message, and the special names ‘body’ or ‘>’ and ‘text’ or ‘=’ will perform full
           text searches – whereas the former searches only the body, the latter also searches
           the message header ([v15 behaviour may differ] this mode yet brute force searches over
           the entire decoded content of messages, including administrativa strings).

           This specification performs full text comparison, but even with regular expression
           support it is almost impossible to write a search expression that safely matches only
           a specific address domain.  To request that the body content of the header is treated
           as a list of addresses, and to strip those down to the plain email address which the
           search expression is to be matched against, prefix the effective name-list with a
           tilde ‘~’:

                 '@~f,c@@a\.safe\.domain\.match$'

     :c    All messages of state or with matching condition ‘c’, where ‘c’ is one or multiple of
           the following colon modifiers:

           a   answered messages (cf. the variable markanswered).
           d   ‘deleted’ messages (for the undelete and from commands only).
           f   flagged messages.
           L   Messages with receivers that match mlsubscribed addresses.
           l   Messages with receivers that match mlisted addresses.
           n   ‘new’ messages.
           o   Old messages (any not in state ‘read’ or ‘new’).
           r   ‘read’ messages.
           S   [Option] Messages with unsure spam classification (see Handling spam).
           s   [Option] Messages classified as spam.
           t   Messages marked as draft.
           u   ‘unread’ messages.

     [Option] IMAP-style SEARCH expressions may also be used.  These consist of keywords and
     criterions, and because Message list arguments are split into tokens according to
     Shell-style argument quoting it is necessary to quote the entire IMAP search expression in
     order to ensure that it remains a single token.  This addressing mode is available with all
     types of mailbox folders; S-nail will perform the search locally as necessary.  Strings must
     be enclosed by double quotes ‘"’ in their entirety if they contain whitespace or
     parentheses; within the quotes, only reverse solidus ‘\’ is recognized as an escape
     character.  All string searches are case-insensitive.  When the description indicates that
     the “envelope” representation of an address field is used, this means that the search string
     is checked against both a list constructed as

           '("name" "source" "local-part" "domain-part")'

     for each address, and the addresses without real names from the respective header field.
     These search expressions can be nested using parentheses, see below for examples.

     (criterion)
           All messages that satisfy the given criterion.
     (criterion1 criterion2 ... criterionN)
           All messages that satisfy all of the given criteria.
     (or criterion1 criterion2)
           All messages that satisfy either criterion1 or criterion2, or both.  To connect more
           than two criteria using ‘or’ specifications have to be nested using additional
           parentheses, as with ‘(or a (or b c))’, since ‘(or a b c)’ really means ‘((a or b) and
           c)’.  For a simple ‘or’ operation of independent criteria on the lowest nesting level,
           it is possible to achieve similar effects by using three separate criteria, as with
           ‘(a) (b) (c)’.
     (not criterion)
           All messages that do not satisfy criterion.
     (bcc "string")
           All messages that contain string in the envelope representation of the ‘Bcc:’ field.
     (cc "string")
           All messages that contain string in the envelope representation of the ‘Cc:’ field.
     (from "string")
           All messages that contain string in the envelope representation of the ‘From:’ field.
     (subject "string")
           All messages that contain string in the ‘Subject:’ field.
     (to "string")
           All messages that contain string in the envelope representation of the ‘To:’ field.
     (header name "string")
           All messages that contain string in the specified ‘Name:’ field.
     (body "string")
           All messages that contain string in their body.
     (text "string")
           All messages that contain string in their header or body.
     (larger size)
           All messages that are larger than size (in bytes).
     (smaller size)
           All messages that are smaller than size (in bytes).
     (before date)
           All messages that were received before date, which must be in the form
           ‘d[d]-mon-yyyy’, where ‘d’ denotes the day of the month as one or two digits, ‘mon’ is
           the name of the month – one of ‘Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec’, and
           ‘yyyy’ is the year as four digits, e.g., ‘28-Dec-2012’.
     (on date)
           All messages that were received on the specified date.
     (since date)
           All messages that were received since the specified date.
     (sentbefore date)
           All messages that were sent on the specified date.
     (senton date)
           All messages that were sent on the specified date.
     (sentsince date)
           All messages that were sent since the specified date.
     ()    The same criterion as for the previous search.  This specification cannot be used as
           part of another criterion.  If the previous command line contained more than one
           independent criterion then the last of those criteria is used.

   On terminal control and line editor
     [Option] Terminal control will be realized through one of the standard UNIX libraries,
     either the Termcap Access Library (libtermcap, -ltermcap), or, alternatively, the Terminal
     Information Library (libterminfo, -lterminfo), both of which will be initialized to work
     with the environment variable TERM.  Terminal control will enhance or enable interactive
     usage aspects, e.g., Coloured display, and extend behaviour of the Mailx-Line-Editor (MLE),
     which may learn the byte-sequences of keys like the cursor- and function-keys.

     The internal variable termcap can be used to overwrite settings or to learn (correct(ed))
     keycodes.  Actual library interaction can be disabled completely by setting termcap-disable;
     termcap will be queried regardless, which is true even if the [Option]al library support has
     not been enabled at configuration time as long as some other [Option] which (may) query
     terminal control sequences has been enabled.  S-nail can be told to enter an alternative
     exclusive screen, the so-called ca-mode, by setting termcap-ca-mode; this requires
     sufficient terminal support, and the used PAGER may also need special configuration,
     dependent on the value of crt.

     [Option] The built-in Mailx-Line-Editor (MLE) should work in all environments which comply
     to the ISO C standard ISO/IEC 9899/AMD1:1995 (“ISO C90, Amendment 1”), and will support wide
     glyphs if possible (the necessary functionality had been removed from ISO C, but was
     included in X/Open Portability Guide Issue 4 (“XPG4”)).  Usage of a line editor in
     interactive mode can be prevented by setting line-editor-disable.  Especially if the
     [Option]al terminal control support is missing setting entries in the internal variable
     termcap will help shall the MLE misbehave, see there for more.  The MLE can support a little
     bit of colour.

     [Option] If the history feature is available then input from line editor prompts will be
     saved in a history list that can be searched in and be expanded from.  Such saving can be
     prevented by prefixing input with any amount of whitespace.  Aspects of history, like
     allowed content and maximum size, as well as whether history shall be saved persistently,
     can be configured with the internal variables history-file, history-gabby,
     history-gabby-persist and history-size.

     The MLE supports a set of editing and control commands.  By default (as) many (as possible)
     of these will be assigned to a set of single-letter control codes, which should work on any
     terminal (and can be generated by holding the “control” key while pressing the key of
     desire, e.g., ‘control-D’).  If the [Option]al bind command is available then the MLE
     commands can also be accessed freely by assigning the command name, which is shown in
     parenthesis in the list below, to any desired key-sequence, and the MLE will instead and
     also use bind to establish its built-in key bindings (more of them if the [Option]al
     terminal control is available), an action which can then be suppressed completely by setting
     line-editor-no-defaults.  Shell-style argument quoting notation is used in the following;
     combinations not mentioned either cause job control signals or do not generate a (unique)
     keycode:

     ‘\cA’  Go to the start of the line (mle-go-home).
     ‘\cB’  Move the cursor backward one character (mle-go-bwd).
     ‘\cD’  Forward delete the character under the cursor; quits S-nail if used on the empty line
            unless the internal variable ignoreeof is set (mle-del-fwd).
     ‘\cE’  Go to the end of the line (mle-go-end).
     ‘\cF’  Move the cursor forward one character (mle-go-fwd).
     ‘\cG’  Cancel current operation, full reset.  If there is an active history search or
            tabulator expansion then this command will first reset that, reverting to the former
            line content; thus a second reset is needed for a full reset in this case
            (mle-reset).
     ‘\cH’  Backspace: backward delete one character (mle-del-bwd).
     ‘\cI’  [Only new quoting rules] Horizontal tabulator: try to expand the word before the
            cursor, supporting the usual Filename transformations (mle-complete; this is affected
            by mle-quote-rndtrip).
     ‘\cJ’  Newline: commit the current line (mle-commit).
     ‘\cK’  Cut all characters from the cursor to the end of the line (mle-snarf-end).
     ‘\cL’  Repaint the line (mle-repaint).
     ‘\cN’  [Option] Go to the next history entry (mle-hist-fwd).
     ‘\cO’  ([Option]ally context-dependent) Invokes the command dt.
     ‘\cP’  [Option] Go to the previous history entry (mle-hist-bwd).
     ‘\cQ’  Toggle roundtrip mode shell quotes, where produced, on and off (mle-quote-rndtrip).
            This setting is temporary, and will be forgotten once the command line is committed;
            also see shcodec.
     ‘\cR’  [Option] Complete the current line from (the remaining) older history entries
            (mle-hist-srch-bwd).
     ‘\cS’  [Option] Complete the current line from (the remaining) newer history entries
            (mle-hist-srch-fwd).
     ‘\cT’  Paste the snarf buffer (mle-paste).
     ‘\cU’  The same as ‘\cA’ followed by ‘\cK’ (mle-snarf-line).
     ‘\cV’  Prompts for a Unicode character (hexadecimal number without prefix, see vexpr) to be
            inserted (mle-prompt-char).  Note this command needs to be assigned to a single-
            letter control code in order to become recognized and executed during input of a key-
            sequence (only three single-letter control codes can be used for that shortcut
            purpose); this control code is then special-treated and thus cannot be part of any
            other sequence (because it will trigger the mle-prompt-char function immediately).
     ‘\cW’  Cut the characters from the one preceding the cursor to the preceding word boundary
            (mle-snarf-word-bwd).
     ‘\cX’  Move the cursor forward one word boundary (mle-go-word-fwd).
     ‘\cY’  Move the cursor backward one word boundary (mle-go-word-bwd).
     ‘\c[’  Escape: reset a possibly used multibyte character input state machine and
            [Option]ally a lingering, incomplete key binding (mle-cancel).  This command needs to
            be assigned to a single-letter control code in order to become recognized and
            executed during input of a key-sequence (only three single-letter control codes can
            be used for that shortcut purpose).  This control code may also be part of a multi-
            byte sequence, but if a sequence is active and the very control code is currently
            also an expected input, then the active sequence takes precedence and will consume
            the control code.
     ‘\c\’  ([Option]ally context-dependent) Invokes the command ‘z+’.
     ‘\c]’  ([Option]ally context-dependent) Invokes the command ‘z$’.
     ‘\c^’  ([Option]ally context-dependent) Invokes the command ‘z0’.
     ‘\c_’  Cut the characters from the one after the cursor to the succeeding word boundary
            (mle-snarf-word-fwd).
     ‘\c?’  Backspace: mle-del-bwd.
     –      Move the cursor forward one screen width (mle-go-screen-fwd).
     –      Move the cursor backward one screen width (mle-go-screen-bwd).
     –      [Option] Move the cursor home and clear the screen (mle-clear-screen).
     –      mle-fullreset: different to mle-reset this will immediately reset a possibly active
            search etc.
     –      mle-bell: ring the audible bell.

   Coloured display
     [Option] S-nail can be configured to support a coloured display and font attributes by
     emitting ANSI a.k.a. ISO 6429 SGR (select graphic rendition) escape sequences.  Usage of
     colours and font attributes solely depends upon the capability of the detected terminal type
     that is defined by the environment variable TERM and which can be fine-tuned by the user via
     the internal variable termcap.

     On top of what S-nail knows about the terminal the boolean variable colour-pager defines
     whether the actually applicable colour and font attribute sequences should also be generated
     when output is going to be paged through the external program defined by the environment
     variable PAGER (also see crt).  This is not enabled by default because different pager
     programs need different command line switches or other configuration in order to support
     those sequences.  S-nail however knows about some widely used pagers and in a clean
     environment it is often enough to simply set colour-pager; please refer to that variable for
     more on this topic.

     Colours and font attributes can be managed with the multiplexer command colour, and uncolour
     can be used to remove mappings of a given colour type.  If the variable colour-disable is
     set then any active usage of colour and font attribute sequences is suppressed without
     affecting possibly established colour mappings.  Since colours are only available in
     interactive mode, it may make sense to conditionalize the colour setup by encapsulating it
     with if:

           if terminal && [ "$features" =% +colour ]
             colour iso view-msginfo ft=bold,fg=green
             colour iso view-header ft=bold,fg=red from,subject
             colour iso view-header fg=red

             uncolour iso view-header from,subject
             colour iso view-header ft=bold,fg=magenta,bg=cyan
             colour 256 view-header ft=bold,fg=208,bg=230 "subject,from"
             colour mono view-header ft=bold
             colour mono view-header ft=bold,ft=reverse subject,from
           endif

   Handling spam
     [Option] S-nail can make use of several spam interfaces for the purpose of identification
     of, and, in general, dealing with spam messages.  A precondition of most commands in order
     to function is that the spam-interface variable is set to one of the supported interfaces.
     Specifying messages that have been identified as spam is possible via their (volatile)
     ‘is-spam’ state by using the ‘:s’ and ‘:S’ specifications, and their attrlist entries will
     be used when displaying the headline in the summary of headers.

     ·   spamrate rates the given messages and sets their ‘is-spam’ flag accordingly.  If the
         spam interface offers spam scores these can be shown in headline by using the format
         ‘%$’.

     ·   spamham, spamspam and spamforget will interact with the Bayesian filter of the chosen
         interface and learn the given messages as “ham” or “spam”, respectively; the last
         command can be used to cause “unlearning” of messages; it adheres to their current
         ‘is-spam’ state and thus reverts previous teachings.

     ·   spamclear and spamset will simply set and clear, respectively, the mentioned volatile
         ‘is-spam’ message flag, without any interface interaction.

     The spamassassin(1) based spam-interface ‘spamc’ requires a running instance of the spamd(1)
     server in order to function, started with the option --allow-tell shall Bayesian filter
     learning be possible.

           $ spamd -i localhost:2142 -i /tmp/.spamsock -d [-L] [-l]
           $ spamd --listen=localhost:2142 --listen=/tmp/.spamsock \
               --daemonize [--local] [--allow-tell]

     Thereafter S-nail can make use of these interfaces:

           $ s-nail -Sspam-interface=spamc -Sspam-maxsize=500000 \
               -Sspamc-command=/usr/local/bin/spamc \
               -Sspamc-arguments="-U /tmp/.spamsock" -Sspamc-user=
           or
           $ s-nail -Sspam-interface=spamc -Sspam-maxsize=500000 \
               -Sspamc-command=/usr/local/bin/spamc \
               -Sspamc-arguments="-d localhost -p 2142" -Sspamc-user=

     Using the generic filter approach allows usage of programs like bogofilter(1).  Here is an
     example, requiring it to be accessible via PATH:

           $ s-nail -Sspam-interface=filter -Sspam-maxsize=500000 \
               -Sspamfilter-ham="bogofilter -n" \
               -Sspamfilter-noham="bogofilter -N" \
               -Sspamfilter-nospam="bogofilter -S" \
               -Sspamfilter-rate="bogofilter -TTu 2>/dev/null" \
               -Sspamfilter-spam="bogofilter -s" \
               -Sspamfilter-rate-scanscore="1;^(.+)$"

     Because messages must exist on local storage in order to be scored (or used for Bayesian
     filter training), it is possibly a good idea to perform the local spam check last.  Spam can
     be checked automatically when opening specific folders by setting a specialized form of the
     internal variable folder-hook.

           define spamdelhook {
             # Server side DCC
             spamset (header x-dcc-brand-metrics "bulk")
             # Server-side spamassassin(1)
             spamset (header x-spam-flag "YES")
             del :s # TODO we HAVE to be able to do `spamrate :u ! :sS'
             move :S +maybe-spam
             spamrate :u
             del :s
             move :S +maybe-spam
           }
           set folder-hook-SOMEFOLDER=spamdelhook

     See also the documentation for the variables spam-interface, spam-maxsize, spamc-command,
     spamc-arguments, spamc-user, spamfilter-ham, spamfilter-noham, spamfilter-nospam,
     spamfilter-rate and spamfilter-rate-scanscore.

COMMANDS

     S-nail reads input in lines.  An unquoted reverse solidus ‘\’ at the end of a command line
     “escapes” the newline character: it is discarded and the next line of input is used as a
     follow-up line, with all leading whitespace removed; once an entire line is completed, the
     whitespace characters space, tabulator, newline as well as those defined by the variable ifs
     are removed from the beginning and end.  Placing any whitespace characters at the beginning
     of a line will prevent a possible addition of the command line to the [Option]al history.

     The beginning of such input lines is then scanned for the name of a known command: command
     names may be abbreviated, in which case the first command that matches the given prefix will
     be used.  Command modifiers may prefix a command in order to modify its behaviour.  A name
     may also be a commandalias, which will become expanded until no more expansion is possible.
     Once the command that shall be executed is known, the remains of the input line will be
     interpreted according to command-specific rules, documented in the following.

     This behaviour is different to the sh(1)ell, which is a programming language with syntactic
     elements of clearly defined semantics, and therefore capable to sequentially expand and
     evaluate individual elements of a line.  S-nail will never be able to handle ‘? set
     one=value two=$one’ in a single statement, because the variable assignment is performed by
     the command (set), not the language.

     The command list can be used to show the list of all commands, either alphabetically sorted
     or in prefix search order (these do not match, also because the POSIX standard prescribes a
     set of abbreviations).  [Option]ally the command help (or ?), when given an argument, will
     show a documentation string for the command matching the expanded argument, as in ‘?t’,
     which should be a shorthand of ‘?type’; with these documentation strings both commands
     support a more verbose listing mode which includes the argument type of the command and
     other information which applies; a handy suggestion might thus be:

           ? define __xv {
             # Before v15: need to enable sh(1)ell-style on _entire_ line!
             localopts yes;wysh set verbose;ignerr eval "${@}";return ${?}
           }
           ? commandalias xv '\call __xv'
           ? xv help set

   Command modifiers
     Commands may be prefixed by one or multiple command modifiers.  Some command modifiers can
     be used with a restricted set of commands only, the verbose version of list will
     ([Option]ally) show which modifiers apply.

     ·   The modifier reverse solidus \, to be placed first, prevents commandalias expansions on
         the remains of the line, e.g., ‘\echo’ will always evaluate the command echo, even if an
         (command)alias of the same name exists.  commandalias content may itself contain further
         command modifiers, including an initial reverse solidus to prevent further expansions.

     ·   The modifier ignerr indicates that any error generated by the following command should
         be ignored by the state machine and not cause a program exit with enabled errexit or for
         the standardized exit cases in posix mode.  ?, one of the INTERNAL VARIABLES, will be
         set to the real exit status of the command regardless.

     ·   local will alter the called command to apply changes only temporarily, local to block-
         scope, and can thus only be used inside of a defined macro or an account definition.
         Specifying it implies the modifier wysh.  Block-scope settings will not be inherited by
         macros deeper in the call chain, and will be garbage collected once the current block is
         left.  To record and unroll changes in the global scope use the command localopts.

     ·   scope does yet not implement any functionality.

     ·   u does yet not implement any functionality.

     ·   Some commands support the vput modifier: if used, they expect the name of a variable,
         which can itself be a variable, i.e., shell expansion is applied, as their first
         argument, and will place their computation result in it instead of the default location
         (it is usually written to standard output).

         The given name will be tested for being a valid sh(1) variable name, and may therefore
         only consist of upper- and lowercase characters, digits, and the underscore; the hyphen-
         minus may be used as a non-portable extension; digits may not be used as first, hyphen-
         minus may not be used as last characters.  In addition the name may either not be one of
         the known INTERNAL VARIABLES, or must otherwise refer to a writable (non-boolean) value
         variable.  The actual put operation may fail nonetheless, e.g., if the variable expects
         a number argument only a number will be accepted.  Any error during these operations
         causes the command as such to fail, and the error number ! will be set to ^ERR-NOTSUP,
         the exit status ? should be set to ‘-1’, but some commands deviate from the latter,
         which is documented.

     ·   Last, but not least, the modifier wysh can be used for some old and established commands
         to choose the new Shell-style argument quoting rules over the traditional Old-style
         argument quoting.

   Old-style argument quoting
     [v15 behaviour may differ] This section documents the old, traditional style of quoting non-
     message-list arguments to commands which expect this type of arguments: whereas still used
     by the majority of such commands, the new Shell-style argument quoting may be available even
     for those via wysh, one of the Command modifiers.  Nonetheless care must be taken, because
     only new commands have been designed with all the capabilities of the new quoting rules in
     mind, which can, e.g., generate control characters.

           ·   An argument can be enclosed between paired double-quotes ‘"argument"’ or single-
               quotes ‘'argument'’; any whitespace, shell word expansion, or reverse solidus
               characters (except as described next) within the quotes are treated literally as
               part of the argument.  A double-quote will be treated literally within single-
               quotes and vice versa.  Inside such a quoted string the actually used quote
               character can be used nonetheless by escaping it with a reverse solidus ‘\’, as in
               ‘"y\"ou"’.

           ·   An argument that is not enclosed in quotes, as above, can usually still contain
               space characters if those spaces are reverse solidus escaped, as in ‘you\ are’.

           ·   A reverse solidus outside of the enclosing quotes is discarded and the following
               character is treated literally as part of the argument.

   Shell-style argument quoting
     sh(1)ell-style, and therefore POSIX standardized, argument parsing and quoting rules are
     used by most commands.  [v15 behaviour may differ] Most new commands only support these new
     rules and are flagged [Only new quoting rules], some elder ones can use them with the
     command modifier wysh; in the future only this type of argument quoting will remain.

     A command line is parsed from left to right and an input token is completed whenever an
     unquoted, otherwise ignored, metacharacter is seen.  Metacharacters are vertical bar |,
     ampersand &, semicolon ;, as well as all characters from the variable ifs, and / or space,
     tabulator, newline.  The additional metacharacters left and right parenthesis (, ) and less-
     than and greater-than signs <, > that the sh(1) supports are not used, and are treated as
     ordinary characters: for one these characters are a vivid part of email addresses, and it
     seems highly unlikely that their function will become meaningful to S-nail.

           Compatibility  note:  [v15  behaviour may differ] Please note that even many new-style
           commands do not yet honour ifs to parse their arguments: whereas  the  sh(1)ell  is  a
           language  with  syntactic  elements of clearly defined semantics, S-nail parses entire
           input lines and decides on a per-command base what to do with the rest  of  the  line.
           This  also  means  that  whenever an unknown command is seen all that S-nail can do is
           cancellation of the processing of the remains of the line.

           It also often depends on an actual subcommand of a multiplexer command how the rest of
           the  line  should  be  treated,  and until v15 we are not capable to perform this deep
           inspection of arguments.  Nonetheless, at least the following commands which work with
           positional   parameters  fully  support  ifs  for  an  almost  shell-compatible  field
           splitting: call, call_if, read, vpospar, xcall.

     Any unquoted number sign ‘#’ at the beginning of a new token starts a comment that extends
     to the end of the line, and therefore ends argument processing.  An unquoted dollar sign ‘$’
     will cause variable expansion of the given name, which must be a valid sh(1)ell-style
     variable name (see vput): INTERNAL VARIABLES as well as ENVIRONMENT (shell) variables can be
     accessed through this mechanism, brace enclosing the name is supported (i.e., to subdivide a
     token).

     Whereas the metacharacters space, tabulator, newline only complete an input token, vertical
     bar |, ampersand & and semicolon ; also act as control operators and perform control
     functions.  For now supported is semicolon ;, which terminates a single command, therefore
     sequencing the command line and making the remainder of the line a subject to reevaluation.
     With sequencing, multiple command argument types and quoting rules may therefore apply to a
     single line, which can become problematic before v15: e.g., the first of the following will
     cause surprising results.

           ? echo one; set verbose; echo verbose=$verbose.
           ? echo one; wysh set verbose; echo verbose=$verbose.

     Quoting is a mechanism that will remove the special meaning of metacharacters and reserved
     words, and will prevent expansion.  There are four quoting mechanisms: the escape character,
     single-quotes, double-quotes and dollar-single-quotes:

           ·   The literal value of any character can be preserved by preceding it with the
               escape character reverse solidus ‘\’.

           ·   Arguments which are enclosed in ‘'single-quotes'’ retain their literal value.  A
               single-quote cannot occur within single-quotes.

           ·   The literal value of all characters enclosed in ‘"double-quotes"’ is retained,
               with the exception of dollar sign ‘$’, which will cause variable expansion, as
               above, backquote (grave accent) ‘`’, (which not yet means anything special),
               reverse solidus ‘\’, which will escape any of the characters dollar sign ‘$’ (to
               prevent variable expansion), backquote (grave accent) ‘`’, double-quote ‘"’ (to
               prevent ending the quote) and reverse solidus ‘\’ (to prevent escaping, i.e., to
               embed a reverse solidus character as-is), but has no special meaning otherwise.

           ·   Arguments enclosed in ‘$'dollar-single-quotes'’ extend normal single quotes in
               that reverse solidus escape sequences are expanded as follows:

               ‘\a’    bell control character (ASCII and ISO-10646 BEL).
               ‘\b’    backspace control character (ASCII and ISO-10646 BS).
               ‘\E’    escape control character (ASCII and ISO-10646 ESC).
               ‘\e’    the same.
               ‘\f’    form feed control character (ASCII and ISO-10646 FF).
               ‘\n’    line feed control character (ASCII and ISO-10646 LF).
               ‘\r’    carriage return control character (ASCII and ISO-10646 CR).
               ‘\t’    horizontal tabulator control character (ASCII and ISO-10646 HT).
               ‘\v’    vertical tabulator control character (ASCII and ISO-10646 VT).
               ‘\\’    emits a reverse solidus character.
               ‘\'’    single quote.
               ‘\"’    double quote (escaping is optional).
               ‘\NNN’  eight-bit byte with the octal value ‘NNN’ (one to three octal digits),
                       optionally prefixed by an additional ‘0’.  A 0 byte will suppress further
                       output for the quoted argument.
               ‘\xHH’  eight-bit byte with the hexadecimal value ‘HH’ (one or two hexadecimal
                       characters, no prefix, see vexpr).  A 0 byte will suppress further output
                       for the quoted argument.
               ‘\UHHHHHHHH’
                       the Unicode / ISO-10646 character with the hexadecimal codepoint value
                       ‘HHHHHHHH’ (one to eight hexadecimal characters) — note that Unicode
                       defines the maximum codepoint ever to be supported as ‘0x10FFFF’ (in
                       planes of ‘0xFFFF’ characters each).  This escape is only supported in
                       locales that support Unicode (see Character sets), in other cases the
                       sequence will remain unexpanded unless the given code point is ASCII
                       compatible or (if the [Option]al character set conversion is available)
                       can be represented in the current locale.  The character NUL will suppress
                       further output for the quoted argument.
               ‘\uHHHH’
                       Identical to ‘\UHHHHHHHH’ except it takes only one to four hexadecimal
                       characters.
               ‘\cX’   Emits the non-printable (ASCII and compatible) C0 control codes 0 (NUL) to
                       31 (US), and 127 (DEL).  Printable representations of ASCII control codes
                       can be created by mapping them to a different, visible part of the ASCII
                       character set.  Adding the number 64 achieves this for the codes 0 to 31,
                       e.g., 7 (BEL): ‘7 + 64 = 71 = G’.  The real operation is a bitwise logical
                       XOR with 64 (bit 7 set, see vexpr), thus also covering code 127 (DEL),
                       which is mapped to 63 (question mark): ‘? vexpr ^ 127 64’.

                       Whereas historically circumflex notation has often been used for
                       visualization purposes of control codes, e.g., ‘^G’, the reverse solidus
                       notation has been standardized: ‘\cG’.  Some control codes also have
                       standardized (ISO-10646, ISO C) aliases, as shown above (e.g., ‘\a’, ‘\n’,
                       ‘\t’): whenever such an alias exists it will be used for display purposes.
                       The control code NUL (‘\c@’, a non-standard extension) will suppress
                       further output for the remains of the token (which may extend beyond the
                       current quote), or, depending on the context, the remains of all arguments
                       for the current command.
               ‘\$NAME’
                       Non-standard extension: expand the given variable name, as above.  Brace
                       enclosing the name is supported.
               ‘\`{command}’
                       Not yet supported, just to raise awareness: Non-standard extension.

     Caveats:

           ? echo 'Quotes '${HOME}' and 'tokens" differ!"# no comment
           ? echo Quotes ${HOME} and tokens differ! # comment
           ? echo Don"'"t you worry$'\x21' The sun shines on us. $'\u263A'

   Message list arguments
     Many commands operate on message list specifications, as documented in Specifying messages.
     The argument input is first split into individual tokens via Shell-style argument quoting,
     which are then interpreted as the mentioned specifications.  If no explicit message list has
     been specified, many commands will search for and use the next message forward that
     satisfies the commands' requirements, and if there are no messages forward of the current
     message, the search proceeds backwards; if there are no good messages at all to be found, an
     error message is shown and the command is aborted.  The verbose output of the command list
     will indicate whether a command searches for a default message, or not.

   Raw data arguments for codec commands
     A special set of commands, which all have the string “codec” in their name, e.g., addrcodec,
     shcodec, urlcodec, take raw string data as input, which means that the content of the
     command input line is passed completely unexpanded and otherwise unchanged: like this the
     effect of the actual codec is visible without any noise of possible shell quoting rules
     etc., i.e., the user can input one-to-one the desired or questionable data.  To gain a level
     of expansion, the entire command line can be evaluated first, e.g.,

           ? vput shcodec res encode /usr/Schönes Wetter/heute.txt
           ? echo $res
           $'/usr/Sch\u00F6nes Wetter/heute.txt'
           ? shcodec d $res
           $'/usr/Sch\u00F6nes Wetter/heute.txt'
           ? eval shcodec d $res
           /usr/Schönes Wetter/heute.txt

   Filename transformations
     Filenames, where expected, and unless documented otherwise, are subsequently subject to the
     following filename transformations, in sequence:

           ·   If the given name is a registered shortcut, it will be replaced with the expanded
               shortcut.

           ·   The filename is matched against the following patterns or strings:

               #      (Number sign) is expanded to the previous file.
               %      (Percent sign) is replaced by the invoking user's primary system mailbox,
                      which either is the (itself expandable) inbox if that is set, the
                      standardized absolute pathname indicated by MAIL if that is set, or a
                      built-in compile-time default otherwise.
               %user  Expands to the primary system mailbox of user (and never the value of
                      inbox, regardless of its actual setting).
               &      (Ampersand) is replaced with the invoking user's secondary mailbox, the
                      MBOX.
               +file  Refers to a file in the folder directory (if that variable is set).
               %:filespec Expands to the same value as filespec, but has special meaning when
                      used with, e.g., the command file: the file will be treated as a primary
                      system mailbox by, e.g., the mbox and save commands, meaning that messages
                      that have been read in the current session will be moved to the MBOX
                      mailbox instead of simply being flagged as read.

           ·   Meta expansions may be applied to the resulting filename, as allowed by the
               operation and applicable to the resulting access protocol (also see On URL syntax
               and credential lookup).  For the file-protocol, a leading tilde ‘~’ character will
               be replaced by the expansion of HOME, except when followed by a valid user name,
               in which case the home directory of the given user is used instead.

               A shell expansion as if specified in double-quotes (see Shell-style argument
               quoting) may be applied, so that any occurrence of ‘$VARIABLE’ (or ‘${VARIABLE}’)
               will be replaced by the expansion of the variable, if possible; INTERNAL VARIABLES
               as well as ENVIRONMENT (shell) variables can be accessed through this mechanism.

               Shell pathname wildcard pattern expansions (glob(7)) may be applied as documented.
               If the fully expanded filename results in multiple pathnames and the command is
               expecting only one file, an error results.

               In interactive context, in order to allow simple value acceptance (via “ENTER”),
               arguments will usually be displayed in a properly quoted form, e.g., a file ‘diet\
               is \curd.txt’ may be displayed as ‘'diet\ is \curd.txt'’.

   Commands
     The following commands are available:

     !     Executes the SHELL command which follows, replacing unescaped exclamation marks with
           the previously executed command if the internal variable bang is set.  This command
           supports vput as documented in Command modifiers, and manages the error number !.  A 0
           or positive exit status ? reflects the exit status of the command, negative ones that
           an error happened before the command was executed, or that the program did not exit
           cleanly, but, e.g., due to a signal: the error number is ^ERR-CHILD, then.

           In conjunction with the vput modifier the following special cases exist: a negative
           exit status occurs if the collected data could not be stored in the given variable,
           which is a ^ERR-NOTSUP error that should otherwise not occur.  ^ERR-CANCELED indicates
           that no temporary file could be created to collect the command output at first glance.
           In case of catchable out-of-memory situations ^ERR-NOMEM will occur and S-nail will
           try to store the empty string, just like with all other detected error conditions.

     #     The comment-command causes the entire line to be ignored.  Note: this really is a
           normal command which' purpose is to discard its arguments, not a “comment-start”
           indicating special character, which means that, e.g., trailing comments on a line are
           not possible (except for commands which use Shell-style argument quoting).

     +     Goes to the next message in sequence and types it (like “ENTER”).

     -     Display the preceding message, or the n'th previous message if given a numeric
           argument n.

     =     Shows the message number of the current message (the “dot”) when used without
           arguments, that of the given list otherwise.  Output numbers will be separated from
           each other with the first character of ifs, and followed by the first character of
           if-ws, if that is not empty and not identical to the first.  If that results in no
           separation at all a space character is used.  This command supports vput (see Command
           modifiers), and manages the error number !.

     ?     [Option] Show a brief summary of commands.  [Option] Given an argument a synopsis for
           the command in question is shown instead; commands can be abbreviated in general and
           this command can be used to see the full expansion of an abbreviation including the
           synopsis, try, e.g., ‘?h’, ‘?hel’ and ‘?help’ and see how the output changes.  This
           mode also supports a more verbose output, which will provide the information
           documented for list.

     |     A synonym for the pipe command.

     account, unaccount
           (ac, una) Creates, selects or lists (an) account(s).  Accounts are special
           incarnations of defined macros and group commands and variable settings which together
           usually arrange the environment for the purpose of creating an email account.
           Different to normal macros settings which are covered by localopts – here by default
           enabled! – will not be reverted before the account is changed again.  The special
           account ‘null’ (case-insensitive) always exists, and all but it can be deleted by the
           latter command, and in one operation with the special name ‘*’.  Also for all but it a
           possibly set on-account-cleanup hook is called once they are left.

           Without arguments a listing of all defined accounts is shown.  With one argument the
           given account is activated: the system inbox of that account will be activated (as via
           file), a possibly installed folder-hook will be run, and the internal variable account
           will be updated.  The two argument form is identical to defining a macro as via
           define:

                 account myisp {
                   set folder=~/mail inbox=+syste.mbox record=+sent.mbox
                   set from='(My Name) myname@myisp.example'
                   set mta=smtp://mylogin@smtp.myisp.example
                 }

     addrcodec
           Perform email address codec transformations on raw-data argument, rather according to
           email standards (RFC 5322; [v15 behaviour may differ] will furtherly improve).
           Supports vput (see Command modifiers), and manages the error number !.  The first
           argument must be either [+[+[+]]]e[ncode], d[ecode], s[kin] or skinl[ist] and
           specifies the operation to perform on the rest of the line.

           Decoding will show how a standard-compliant MUA will display the given argument, which
           should be an email address.  Please be aware that most MUAs have difficulties with the
           address standards, and vary wildly when (comments) in parenthesis, “double-quoted”
           strings, or quoted-pairs, as below, become involved.  [v15 behaviour may differ]
           S-nail currently does not perform decoding when displaying addresses.

           Skinning is identical to decoding but only outputs the plain address, without any
           string, comment etc. components.  Another difference is that it may fail with the
           error number ! set to ^ERR-INVAL if decoding fails to find a(n) (valid) email address,
           in which case the unmodified input will be output again.

           skinlist first performs a skin operation, and thereafter checks a valid address for
           whether it is a registered mailing list (see mlist and mlsubscribe), eventually
           reporting that state in the error number ! as ^ERR-EXIST.  (This state could later
           become overwritten by an I/O error, though.)

           Encoding supports four different modes, lesser automated versions can be chosen by
           prefixing one, two or three plus signs: the standard imposes a special meaning on some
           characters, which thus have to be transformed to so-called quoted-pairs by pairing
           them with a reverse solidus ‘\’ in order to remove the special meaning; this might
           change interpretation of the entire argument from what has been desired, however!
           Specify one plus sign to remark that parenthesis shall be left alone, two for not
           turning double quotation marks into quoted-pairs, and three for also leaving any user-
           specified reverse solidus alone.  The result will always be valid, if a successful
           exit status is reported ([v15 behaviour may differ] the current parser fails this
           assertion for some constructs).  [v15 behaviour may differ] Addresses need to be
           specified in between angle brackets ‘<’, ‘>’ if the construct becomes more difficult,
           otherwise the current parser will fail; it is not smart enough to guess right.

                 ? addrc enc "Hey, you",<diet@exam.ple>\ out\ there
                 "\"Hey, you\", \\ out\\ there" <diet@exam.ple>
                 ? addrc d "\"Hey, you\", \\ out\\ there" <diet@exam.ple>
                 "Hey, you", \ out\ there <diet@exam.ple>
                 ? addrc s "\"Hey, you\", \\ out\\ there" <diet@exam.ple>
                 diet@exam.ple

     alias, unalias
           (a, una) Aliases are a method of creating personal distribution lists that map a
           single alias name to none to multiple real receivers; these aliases become expanded
           after message composing is completed.  The latter command removes the given list of
           aliases, the special name ‘*’ will discard all existing aliases.

           The former command shows all currently defined aliases when used without arguments,
           and with one argument the expansion of the given alias.  With more than one argument,
           creates or appends to the alias name given as the first argument the remaining
           arguments.  Alias names adhere to the Postfix MTA aliases(5) rules and are thus
           restricted to alphabetic characters, digits, the underscore, hyphen-minus, the number
           sign, colon and commercial at, a dollar sign is allowed but in the first position; As
           extensions the exclamation mark ‘!’, period ‘.’ as well as “any haracter that has the
           high bit set” may be used: ‘[[:alnum:]_#:@-][[:alnum:]_#:@$;.-]*’.

           [v15 behaviour may differ] Unfortunately the colon is currently not supported, as it
           interferes with normal address parsing rules.  [v15 behaviour may differ] Such high
           bit characters will likely cause warnings at the moment for the same reasons why colon
           is unsupported; also, in the future locale dependent character set validity checks
           will be performed.

     alternates, unalternates
           [Only new quoting rules] (alt) Manage a list of alternate addresses or names of the
           active user, members of which will be removed from recipient lists (except one).
           There is a set of implicit alternates which is formed of the values of LOGNAME, from,
           sender and reply-to.  from will not be used if sender is set.  The latter command
           removes the given list of alternates, the special name ‘*’ will discard all existing
           alternate names.

           The former command manages the error number !.  It shows the current set of alternates
           when used without arguments; in this mode only it also supports vput (see Command
           modifiers).  Otherwise the given arguments (after being checked for validity) are
           appended to the list of alternate names; in posix mode they replace that list instead.

     answered, unanswered
           Take a message lists and mark each message as (not) having been answered.  Messages
           will be marked answered when being replyd to automatically if the markanswered
           variable is set.  See the section Message states.

     bind, unbind
           [Option][Only new quoting rules] The bind command extends the MLE (see On terminal
           control and line editor) with freely configurable key bindings.  The latter command
           removes from the given context the given key binding, both of which may be specified
           as a wildcard ‘*’, so that, e.g., ‘unbind * *’ will remove all bindings of all
           contexts.  Due to initialization order unbinding will not work for built-in key
           bindings upon program startup, however: please use line-editor-no-defaults for this
           purpose instead.

           With one argument the former command shows all key bindings for the given context,
           specifying an asterisk ‘*’ will show the bindings of all contexts; a more verbose
           listing will be produced if either of debug or verbose are set.  With two or more
           arguments a binding is (re)established: the first argument is the context to which the
           binding shall apply, the second argument is a comma-separated list of the “keys” which
           form the binding, and any remaining arguments form the expansion.  To indicate that a
           binding shall not be auto-committed, but that the expansion shall instead be furtherly
           editable by the user, a commercial at ‘@’ (that will be removed) can be placed last in
           the expansion, from which leading and trailing whitespace will finally be removed.
           Reverse solidus cannot be used as the last character of expansion.

           Contexts define when a binding applies, i.e., a binding will not be seen unless the
           context for which it is defined for is currently active.  This is not true for the
           shared binding ‘base’, which is the foundation for all other bindings and as such
           always applies, its bindings, however, only apply secondarily.  The available contexts
           are the shared ‘base’, the ‘default’ context which is used in all not otherwise
           documented situations, and ‘compose’, which applies to compose mode only.

           “Keys” which form the binding are specified as a comma-separated list of byte-
           sequences, where each list entry corresponds to one key(press).  A list entry may,
           indicated by a leading colon character ‘:’, also refer to the name of a terminal
           capability; several dozen names will be compiled in and may be specified either by
           their terminfo(5), or, if existing, by their termcap(5) name, regardless of the
           actually used [Option]al terminal control library.  It is possible to use any
           capability, as long as the name is resolvable by the [Option]al control library or was
           defined via the internal variable termcap.  Input sequences are not case-normalized,
           so that an exact match is required to update or remove a binding.  Examples:

                 ? bind base $'\E',d mle-snarf-word-fwd # Esc(ape)
                 ? bind base $'\E',$'\c?' mle-snarf-word-bwd # Esc,Delete
                 ? bind default $'\cA',:khome,w 'echo Editable binding@'
                 ? bind default a,b,c rm -irf / @  # Also editable
                 ? bind default :kf1 File %
                 ? bind compose :kf1 ~v

           Note that the entire comma-separated list is first parsed (over) as a shell-token with
           whitespace as the field separator, before being parsed and expanded for real with
           comma as the field separator, therefore whitespace needs to be properly quoted, see
           Shell-style argument quoting.  Using Unicode reverse solidus escape sequences renders
           a binding defunctional if the locale does not support Unicode (see Character sets),
           and using terminal capabilities does so if no (corresponding) terminal control support
           is (currently) available.

           The following terminal capability names are built-in and can be used in terminfo(5) or
           (if available) the two-letter termcap(5) notation.  See the respective manual for a
           list of capabilities.  The program infocmp(1) can be used to show all the capabilities
           of TERM or the given terminal type; using the -x flag will also show supported (non-
           standard) extensions.

           kbs or kb       Backspace.
           kdch1 or kD     Delete character.
           kDC or *4       — shifted variant.
           kel or kE       Clear to end of line.
           kext or @9      Exit.
           kich1 or kI     Insert character.
           kIC or #3       — shifted variant.
           khome or kh     Home.
           kHOM or #2      — shifted variant.
           kend or @7      End.
           knp or kN       Next page.
           kpp or kP       Previous page.
           kcub1 or kl     Left cursor (with more modifiers: see below).
           kLFT or #4      — shifted variant.
           kcuf1 or kr     Right cursor (ditto).
           kRIT or %i      — shifted variant.
           kcud1 or kd     Down cursor (ditto).
           kDN             — shifted variant (only terminfo).
           kcuu1 or ku     Up cursor (ditto).
           kUP             — shifted variant (only terminfo).
           kf0 or k0       Function key 0.  Add one for each function key up to kf9 and k9,
                           respectively.
           kf10 or k;      Function key 10.
           kf11 or F1      Function key 11.  Add one for each function key up to kf19 and F9,
                           respectively.

           Some terminals support key-modifier combination extensions, e.g., ‘Alt+Shift+xy’.  For
           example, the delete key, kdch1: in its shifted variant, the name is mutated to kDC,
           then a number is appended for the states ‘Alt’ (kDC3), ‘Shift+Alt’ (kDC4), ‘Control’
           (kDC5), ‘Shift+Control’ (kDC6), ‘Alt+Control’ (kDC7), finally ‘Shift+Alt+Control’
           (kDC8).  The same for the left cursor key, kcub1: KLFT, KLFT3, KLFT4, KLFT5, KLFT6,
           KLFT7, KLFT8.

           It is advisable to use an initial escape or other control character (e.g., ‘\cA’) for
           bindings which describe user key combinations (as opposed to purely terminal
           capability based ones), in order to avoid ambiguities whether input belongs to key
           sequences or not; it also reduces search time.  Adjusting bind-timeout may help shall
           keys and sequences be falsely recognized.

     call  [Only new quoting rules] Calls the given macro, which must have been created via
           define (see there for more), otherwise an ^ERR-NOENT error occurs.  Calling macros
           recursively will at some time excess the stack size limit, causing a hard program
           abortion; if recursively calling a macro is the last command of the current macro,
           consider to use the command xcall, which will first release all resources of the
           current macro before replacing the current macro with the called one.

     call_if
           Identical to call if the given macro has been created via define, but does not fail
           nor warn if the macro does not exist.

     cd    (ch) Change the working directory to HOME or the given argument.  Synonym for chdir.

     certsave
           [Option] Only applicable to S/MIME signed messages.  Takes an optional message list
           and a filename and saves the certificates contained within the message signatures to
           the named file in both human-readable and PEM format.  The certificates can later be
           used to send encrypted messages to the respective message senders by setting
           smime-encrypt-USER@HOST variables.

     charsetalias, uncharsetalias
           [Only new quoting rules] Manage alias mappings for (conversion of) Character sets.
           Mappings are ineffective if character set conversion is not available (features does
           not announce ‘+iconv’).  Expansion happens recursively, but expansion is not performed
           for INTERNAL VARIABLES, e.g., charset-8bit.

           The latter command deletes all aliases given as arguments, all aliases can be deleted
           at once with the special argument ‘*’.  The former shows the list of all currently
           defined aliases if used without arguments, the expansion of the given alias with one
           argument.  Otherwise all given arguments are treated as pairs of character sets and
           their desired target alias name, creating new or changing already existing aliases, as
           necessary.

     chdir
           (ch) Change the working directory to HOME or the given argument.  Synonym for cd.

     collapse, uncollapse
           Only applicable to ‘thread’ed sort mode.  Takes a message list and makes all replies
           to these messages invisible in header summaries, except for ‘new’ messages and the
           “dot”.  Also when a message with collapsed replies is displayed, all of these are
           automatically uncollapsed.  The latter command undoes collapsing.

     colour, uncolour
           [Option][Only new quoting rules] Manage colour mappings of and for a Coloured display.
           The type of colour is given as the (case-insensitive) first argument, which must be
           one of ‘256’ for 256-colour terminals, ‘8’, ‘ansi’ or ‘iso’ for the standard 8-colour
           ANSI / ISO 6429 colour palette and ‘1’ or ‘mono’ for monochrome terminals.  Monochrome
           terminals cannot deal with colours, but only (some) font attributes.

           Without further arguments the list of all currently defined mappings for the given
           colour type is shown (as a special case giving ‘all’ or ‘*’ will show the mappings of
           all types).  Otherwise the second argument defines the mappable slot, and the third
           argument a (comma-separated list of) colour and font attribute specification(s), and
           the optional fourth argument can be used to specify a precondition: if conditioned
           mappings exist they are tested in (creation) order unless a (case-insensitive) match
           has been found, and the default mapping (if any has been established) will only be
           chosen as a last resort.  The types of precondition available depend on the mappable
           slot (see Coloured display for some examples), the following of which exist:

           Mappings prefixed with ‘mle-’ are used for the [Option]al built-in Mailx-Line-Editor
           (MLE, see On terminal control and line editor) and do not support preconditions.

           mle-position   This mapping is used for the position indicator that is visible when a
                          line cannot be fully displayed on the screen.
           mle-prompt     Used for the prompt.

           Mappings prefixed with ‘sum-’ are used in header summaries, and they all understand
           the preconditions ‘dot’ (the current message) and ‘older’ for elder messages (only
           honoured in conjunction with datefield-markout-older).

           sum-dotmark    This mapping is used for the “dotmark” that can be created with the
                          ‘%>’ or ‘%<’ formats of the variable headline.
           sum-header     For the complete header summary line except the “dotmark” and the
                          thread structure.
           sum-thread     For the thread structure which can be created with the ‘%i’ format of
                          the variable headline.

           Mappings prefixed with ‘view-’ are used when displaying messages.

           view-from_     This mapping is used for so-called ‘From_’ lines, which are MBOX file
                          format specific header lines.
           view-header    For header lines.  A comma-separated list of headers to which the
                          mapping applies may be given as a precondition; if the [Option]al
                          regular expression support is available then if any of the “magical”
                          (extended) regular expression characters is seen the precondition will
                          be evaluated as (an extended) one.
           view-msginfo   For the introductional message info line.
           view-partinfo  For MIME part info lines.

           The following (case-insensitive) colour definitions and font attributes are
           understood, multiple of which can be specified in a comma-separated list:

           ft=  a font attribute: ‘bold’, ‘reverse’ or ‘underline’.  It is possible (and often
                applicable) to specify multiple font attributes for a single mapping.

           fg=  foreground colour attribute: ‘black’, ‘blue’, ‘green’, ‘red’, ‘brown’, ‘magenta’,
                ‘cyan’ or ‘white’.  To specify a 256-colour mode a decimal number colour
                specification in the range 0 to 255, inclusive, is supported, and interpreted as
                follows:

                0 - 7      the standard ISO 6429 colours, as above.
                8 - 15     high intensity variants of the standard colours.
                16 - 231   216 colours in tuples of 6.
                232 - 255  grayscale from black to white in 24 steps.

                      #!/bin/sh -
                      fg() { printf "\033[38;5;${1}m($1)"; }
                      bg() { printf "\033[48;5;${1}m($1)"; }
                      i=0
                      while [ $i -lt 256 ]; do fg $i; i=$(($i + 1)); done
                      printf "\033[0m\n"
                      i=0
                      while [ $i -lt 256 ]; do bg $i; i=$(($i + 1)); done
                      printf "\033[0m\n"

           bg=  background colour attribute (see fg= for possible values).

           The command uncolour will remove for the given colour type (the special type ‘*’
           selects all) the given mapping; if the optional precondition argument is given only
           the exact tuple of mapping and precondition is removed.  The special name ‘*’ will
           remove all mappings (no precondition allowed), thus ‘uncolour * *’ will remove all
           established mappings.

     commandalias, uncommandalias
           [Only new quoting rules] Define or list, and remove, respectively, command aliases.
           An (command)alias can be used everywhere a normal command can be used, but always
           takes precedence: any arguments that are given to the command alias are joined onto
           the alias expansion, and the resulting string forms the command line that is, in
           effect, executed.  The latter command removes all given aliases, the special name ‘*’
           will remove all existing aliases.  When used without arguments the former shows a list
           of all currently known aliases, with one argument only the expansion of the given one.

           With two or more arguments a command alias is defined or updated: the first argument
           is the name under which the remaining command line should be accessible, the content
           of which can be just about anything.  An alias may itself expand to another alias, but
           to avoid expansion loops further expansion will be prevented if an alias refers to
           itself or if an expansion depth limit is reached.  Explicit expansion prevention is
           available via reverse solidus \, one of the Command modifiers.

                 ? commandalias xx
                 s-nail: `commandalias': no such alias: xx
                 ? commandalias xx echo hello,
                 ? commandalias xx
                 commandalias xx 'echo hello,'
                 ? xx
                 hello,
                 ? xx world
                 hello, world

     Copy  (C) Copy messages to files whose names are derived from the author of the respective
           message and do not mark them as being saved; otherwise identical to Save.

     copy  (c) Copy messages to the named file and do not mark them as being saved; otherwise
           identical to save.

     cwd   Show the name of the current working directory, as reported by getcwd(3).  Supports
           vput (see Command modifiers).  The return status is tracked via ?.

     Decrypt
           [Option] For unencrypted messages this command is identical to Copy; Encrypted
           messages are first decrypted, if possible, and then copied.

     decrypt
           [Option] For unencrypted messages this command is identical to copy; Encrypted
           messages are first decrypted, if possible, and then copied.

     define, undefine
           The latter command deletes the given macro, the special name ‘*’ will discard all
           existing macros.  Deletion of (a) macro(s) can be performed from within running (a)
           macro(s), including self-deletion.  Without arguments the former command prints the
           current list of macros, including their content, otherwise it defines a macro,
           replacing an existing one of the same name as applicable.

           A defined macro can be invoked explicitly by using the call, call_if and xcall
           commands, or implicitly if a macro hook is triggered, e.g., a folder-hook.  Execution
           of a macro body can be stopped from within by calling return.

           Temporary macro block-scope variables can be created or deleted with the local command
           modifier in conjunction with the commands set and unset, respectively.  To enforce
           unrolling of changes made to (global) INTERNAL VARIABLES the command localopts can be
           used instead; its covered scope depends on how (i.e., “as what”: normal macro, folder
           hook, hook, account switch) the macro is invoked.

           Inside a called macro, the given positional parameters are implicitly local to the
           macro's scope, and may be accessed via the variables *, @, # and 1 and any other
           positive unsigned decimal number less than or equal to #.  Positional parameters can
           be shifted, or become completely replaced, removed etc. via vpospar.  A helpful
           command to perform many sorts of number and string evaluations is vexpr.

                 define name {
                   command1
                   command2
                   ...
                   commandN
                 }

                 # E.g.
                 define exmac {
                   echo Parameter 1 of ${#} is ${1}, all: ${*} / ${@}
                   return 1000 0
                 }
                 call exmac Hello macro exmac!
                 echo ${?}/${!}/${^ERRNAME}

     delete, undelete
           (d, u) Marks the given message list as being or not being ‘deleted’, respectively; if
           no argument has been specified then the usual search for a visible message is
           performed, as documented for Message list arguments, showing only the next input
           prompt if the search fails.  Deleted messages will neither be saved in the secondary
           mailbox MBOX nor will they be available for most other commands.  If the autoprint
           variable is set, the new “dot” or the last message restored, respectively, is
           automatically typed; also see dp, dt.

     digmsg
           [Only new quoting rules] Digging (information out of) messages is possible through
           digmsg objects, which can be created for the given message number; in compose mode the
           hyphen-minus ‘-’ will instead open the message that is being composed.  If a hyphen-
           minus is given as the optional third argument then output will be generated on the
           standard output channel instead of being subject to consumation by the read or readall
           commands.

           The objects may be removed again by giving the same identifier used for creation; this
           step could be omitted: objects will be automatically closed when the active mailbox or
           the compose mode is left, respectively.  In all other cases the second argument is an
           object identifier, and the third and all following arguments are interpreted as via ~^
           (see COMMAND ESCAPES):

                 ? vput = msgno; digmsg create $msgno
                 ? digmsg $msgno header list;   readall x;   echon $x
                 210 Subject From To Message-ID References In-Reply-To Status
                 ? digmsg $msgno header show Status;readall x;echon $x
                 212 Status
                 RO

                 ? digmsg remove $msgno

     discard
           (di) Identical to ignore.  Superseded by the multiplexer headerpick.

     dp, dt
           Delete the given messages and automatically type the new “dot” if one exists,
           regardless of the setting of autoprint.

     dotmove
           Move the “dot” up or down by one message when given ‘+’ or ‘-’ argument, respectively.

     draft, undraft
           Take message lists and mark each given message as being draft, or not being draft,
           respectively, as documented in the section Message states.

     echo  [Only new quoting rules] (ec) Echoes arguments to standard output and writes a
           trailing newline, whereas the otherwise identical echon does not.  Shell-style
           argument quoting is used, Filename transformations are applied to the expanded
           arguments.  This command also supports vput as documented in Command modifiers, and
           manages the error number !: if data is stored in a variable then the return value
           reflects the length of the result string in case of success and is ‘-1’ on error.

     echoerr
           [Only new quoting rules] Identical to echo except that is echoes to standard error.
           Also see echoerrn.  In interactive sessions the [Option]al message ring queue for
           errors will be used instead, if available and vput was not used.

     echon
           [Only new quoting rules] Identical to echo, but does not write or store a trailing
           newline.

     echoerrn
           [Only new quoting rules] Identical to echoerr, but does not write or store a trailing
           newline.

     edit  (e) Point the text EDITOR at each message from the given list in turn.  Modified
           contents are discarded unless the writebackedited variable is set, and are not used
           unless the mailbox can be written to and the editor returns a successful exit status.
           visual can be used instead for a more display oriented editor.

     elif  Part of the if (see there for more), elif, else, endif conditional — if the condition
           of a preceding if was false, check the following condition and execute the following
           block if it evaluates true.

     else  (el) Part of the if (see there for more), elif, else, endif conditional — if none of
           the conditions of the preceding if and elif commands was true, the else block is
           executed.

     endif
           (en) Marks the end of an if (see there for more), elif, else, endif conditional
           execution block.

     environ
           [Only new quoting rules] S-nail has a strict notion about which variables are INTERNAL
           VARIABLES and which are managed in the program ENVIRONMENT.  Since some of the latter
           are a vivid part of S-nails functioning, however, they are transparently integrated
           into the normal handling of internal variables via set and unset.  To integrate other
           environment variables of choice into this transparent handling, and also to export
           internal variables into the process environment where they normally are not, a ‘link’
           needs to become established with this command, as in, e.g.,

                 environ link PERL5LIB TZ

           Afterwards changing such variables with set will cause automatic updates of the
           program environment, and therefore be inherited by newly created child processes.
           Sufficient system support provided (it was in BSD as early as 1987, and is
           standardized since Y2K) removing such variables with unset will remove them also from
           the program environment, but in any way the knowledge they ever have been ‘link’ed
           will be lost.  Note that this implies that localopts may cause loss of such links.

           The command ‘unlink’ will remove an existing link, but leaves the variables as such
           intact.  Additionally the subcommands ‘set’ and ‘unset’ are provided, which work
           exactly the same as the documented commands set and unset, but (additionally un)link
           the variable(s) with the program environment and thus immediately export them to, or
           remove them from (if possible), respectively, the program environment.

     errors
           [Option] Since S-nail uses the console as a user interface it can happen that messages
           scroll by too fast to become recognized.  An error message ring queue is available
           which stores duplicates of any error message and notifies the user in interactive
           sessions whenever a new error has occurred.  The queue is finite: if its maximum size
           is reached any new message replaces the eldest.  The command errors can be used to
           manage this message queue: if given show or no argument the queue will be displayed
           and cleared, clear will only clear all messages from the queue.

     eval  [Only new quoting rules] Construct a command by concatenating the arguments, separated
           with a single space character, and then evaluate the result.  This command passes
           through the exit status ? and error number ! of the evaluated command; also see call.

                 define xxx {
                   echo "xxx arg <$1>"
                   shift
                   if [ $# -gt 0 ]
                     \xcall xxx "$@"
                   endif
                 }
                 define yyy {
                   eval "$@ ' ball"
                 }
                 call yyy '\call xxx' "b\$'\t'u ' "
                 call xxx arg <b      u>
                 call xxx arg <  >
                 call xxx arg <ball>

     exit  (ex or x) Exit from S-nail without changing the active mailbox and skip any saving of
           messages in the secondary mailbox MBOX, as well as a possibly tracked line editor
           history-file.  The optional status number argument will be passed through to exit(3).
           [v15 behaviour may differ] For now it can happen that the given status will be
           overwritten, later this will only occur if a later error needs to be reported onto an
           otherwise success indicating status.

     File  (Fi) Like file, but open the mailbox read-only.

     file  (fi) The file command switches to a new mailbox.  Without arguments it shows status
           information of the current mailbox.  If an argument is given, it will write out
           changes (such as deletions) the user has made, open a new mailbox, update the internal
           variables mailbox-resolved and mailbox-display, execute an according folder-hook, if
           one is installed, and optionally display a summary of headers if the variable header
           is set.

           Filename transformations will be applied to the name argument, and ‘protocol://’
           prefixes are, i.e., URL syntax is understood, e.g., ‘mbox:///tmp/mdirbox’: if a
           protocol prefix is used the mailbox type is fixated and neither the auto-detection
           (read on) nor the newfolders mechanisms apply.  [Option]ally URLs can also be used to
           access network resources, which may be accessed securely via Encrypted network
           communication if so supported, and it is possible to proxy all network traffic over a
           SOCKS5 server given via socks-proxy.

                 [v15-compat] protocol://[user[:password]@]host[:port][/path]
                 [no v15-compat] protocol://[user@]host[:port][/path]

           [Option]ally supported network protocols are pop3 (POP3) and pop3s (POP3 with TLS
           encrypted transport), imap and imaps.  The [/path] part is valid only for IMAP; there
           it defaults to INBOX.  Network URLs require a special encoding as documented in the
           section On URL syntax and credential lookup.

           If the resulting file protocol (MBOX database) name is located on a local filesystem
           then the list of all registered filetypes is traversed in order to see whether a
           transparent intermediate conversion step is necessary to handle the given mailbox, in
           which case S-nail will use the found hook to load and save data into and from a
           temporary file, respectively.  Changing hooks will not affect already opened
           mailboxes.  For example, the following creates hooks for the gzip(1) compression tool
           and a combined compressed and encrypted format:

                 ? filetype \
                     gzip 'gzip -dc' 'gzip -c' \
                     zst.pgp 'gpg -d | zstd -dc' 'zstd -19 -zc | gpg -e'

           MBOX databases will always be protected via file-region locks (fcntl(2)) during file
           operations in order to avoid inconsistencies due to concurrent modifications.
           [Option] In addition mailbox files treated as the system inbox (MAIL), as well as
           primary system mailboxes in general will also be protected by so-called dotlock files,
           the traditional way of mail spool file locking: for any file ‘x’ a lock file ‘x.lock’
           will be created for the duration of the synchronization — as necessary an external
           privileged dotlock helper will be used to create the dotlock file in the same
           directory and with the same user and group identities as the file of interest.
           dotlock-disable can be used to turn off additional dotlock files, shall the need
           arise.

           S-nail by default uses tolerant POSIX rules when reading MBOX database files, but it
           will detect invalid message boundaries in this mode and complain (even more with
           debug) if any is seen: in this case mbox-rfc4155 can be used to create a valid MBOX
           database from the invalid input.

           [Option] If no protocol has been fixated, and name refers to a directory with the
           subdirectories ‘tmp’, ‘new’ and ‘cur’, then it is treated as a folder in “Maildir”
           format.  The maildir format stores each message in its own file, and has been designed
           so that file locking is not necessary when reading or writing files.

           [v15 behaviour may differ] If no protocol has been fixated and no existing file has
           been found, the variable newfolders controls the format of mailboxes yet to be
           created.

     filetype, unfiletype
           [Only new quoting rules] Define or list, and remove, respectively, file handler hooks,
           which provide (shell) commands that enable S-nail to load and save MBOX files from and
           to files with the registered file extensions; it will use an intermediate temporary
           file to store the plain data.  The latter command removes the hooks for all given
           extensions, ‘*’ will remove all existing handlers.

           When used without arguments the former shows a list of all currently defined file
           hooks, with one argument the expansion of the given alias.  Otherwise three arguments
           are expected, the first specifying the file extension for which the hook is meant, and
           the second and third defining the load- and save commands, respectively, to deal with
           the file type, both of which must read from standard input and write to standard
           output.  Changing hooks will not affect already opened mailboxes ([v15 behaviour may
           differ] except below).  [v15 behaviour may differ] For now too much work is done, and
           files are oftened read in twice where once would be sufficient: this can cause
           problems if a filetype is changed while such a file is opened; this was already so
           with the built-in support of .gz etc. in Heirloom, and will vanish in v15.  [v15
           behaviour may differ] For now all handler strings are passed to the SHELL for
           evaluation purposes; in the future a ‘!’ prefix to load and save commands may mean to
           bypass this shell instance: placing a leading space will avoid any possible
           misinterpretations.

                 ? filetype bz2 'bzip2 -dc' 'bzip2 -zc' \
                     gz 'gzip -dc' 'gzip -c'  xz 'xz -dc' 'xz -zc' \
                     zst 'zstd -dc' 'zstd -19 -zc' \
                     zst.pgp 'gpg -d | zstd -dc' 'zstd -19 -zc | gpg -e'
                 ? set record=+sent.zst.pgp

     flag, unflag
           Take message lists and mark the messages as being flagged, or not being flagged,
           respectively, for urgent/special attention.  See the section Message states.

     folder
           (fold) The same as file.

     folders
           With no arguments, list the names of the folders in the folder directory.  With an
           existing folder as an argument, lists the names of folders below the named folder.

     Followup
           (F) Similar to Respond, but saves the message in a file named after the local part of
           the first recipient's address (instead of in record).

     followup
           (fo) Similar to respond, but saves the message in a file named after the local part of
           the first recipient's address (instead of in record).

     followupall
           Similar to followup, but responds to all recipients regardless of the flipr variable.

     followupsender
           Similar to Followup, but responds to the sender only regardless of the flipr variable.

     Forward
           Similar to forward, but saves the message in a file named after the local part of the
           recipient's address (instead of in record).

     forward
           Takes a message and the address of a recipient and forwards the message to him.  The
           text of the original message is included in the new one, with the value of the
           forward-inject-head variable preceding, and the value of forward-inject-tail
           succeeding it.  To filter the included header fields to the desired subset use the
           ‘forward’ slot of the white- and blacklisting command headerpick.  Only the first part
           of a multipart message is included unless forward-as-attachment, and recipient
           addresses will be stripped from comments, names etc. unless the internal variable
           fullnames is set.

           This may generate the errors ^ERR-DESTADDRREQ if no receiver has been specified,
           ^ERR-PERM if some addressees where rejected by expandaddr, ^ERR-NODATA if no
           applicable messages have been given, ^ERR-NOTSUP if multiple messages have been
           specified, ^ERR-IO if an I/O error occurs, ^ERR-NOTSUP if a necessary character set
           conversion fails, and ^ERR-INVAL for other errors.

     from  (f) Takes a list of message specifications and displays a summary of their message
           headers, exactly as via headers, making the first message of the result the new “dot”
           (the last message if showlast is set).  An alias of this command is search.  Also see
           Specifying messages.

     Fwd   [Obsolete] Alias for Forward.

     fwd   [Obsolete] Alias for forward.

     fwdignore
           [Obsolete] Superseded by the multiplexer headerpick.

     fwdretain
           [Obsolete] Superseded by the multiplexer headerpick.

     ghost, unghost
           [Obsolete] Replaced by commandalias, uncommandalias.

     headerpick, unheaderpick
           [Only new quoting rules] Multiplexer command to manage white- and blacklisting
           selections of header fields for a variety of applications.  Without arguments the set
           of contexts that have settings is displayed.  When given arguments, the first argument
           is the context to which the command applies, one of (case-insensitive) ‘type’ for
           display purposes (via, e.g., type), ‘save’ for selecting which headers shall be stored
           persistently when save, copy, move or even decrypting messages (note that MIME related
           etc. header fields should not be ignored in order to not destroy usability of the
           message in this case), ‘forward’ for stripping down messages when forwarding message
           (has no effect if forward-as-attachment is set), and ‘top’ for defining user-defined
           set of fields for the command top.

           The current settings of the given context are displayed if it is the only argument.  A
           second argument denotes the type of restriction that is to be chosen, it may be (a
           case-insensitive prefix of) ‘retain’ or ‘ignore’ for white- and blacklisting purposes,
           respectively.  Establishing a whitelist suppresses inspection of the corresponding
           blacklist.

           If no further argument is given the current settings of the given type will be
           displayed, otherwise the remaining arguments specify header fields, which [Option]ally
           may be given as regular expressions, to be added to the given type.  The special
           wildcard field (asterisk, ‘*’) will establish a (fast) shorthand setting which covers
           all fields.

           The latter command always takes three or more arguments and can be used to remove
           selections, i.e., from the given context, the given type of list, all the given
           headers will be removed, the special argument ‘*’ will remove all headers.

     headers
           (h) Show the current group of headers, the size of which depends on the variable
           screen in interactive mode, and the format of which can be defined with headline.  If
           a message-specification is given the group of headers containing the first message
           therein is shown and the message at the top of the screen becomes the new “dot”; the
           last message is targeted if showlast is set.

     help  (hel) A synonym for ?.

     history
           [Option] Without arguments or when given show all history entries are shown (this mode
           also supports a more verbose output).  load will replace the list of entries with the
           content of history-file, and save will dump the current list to said file, replacing
           former content.  clear will delete all history entries.  The argument can also be a
           signed decimal NUMBER, which will select and evaluate the respective history entry,
           and move it to the top of the history; a negative number is used as an offset to the
           current command, e.g., ‘-1’ will select the last command, the history top.  Please see
           On terminal control and line editor for more on this topic.

     hold  (ho, also preserve) Takes a message list and marks each message therein to be saved in
           the user's system inbox instead of in the secondary mailbox MBOX.  Does not override
           the delete command.  S-nail deviates from the POSIX standard with this command,
           because a next command issued after hold will display the following message, not the
           current one.

     if    (i) Part of the if, elif, else, endif conditional execution construct — if the given
           condition is true then the encapsulated block is executed.  The POSIX standard only
           supports the (case-insensitive) conditions ‘r’eceive and ‘s’end, the remaining are
           non-portable extensions.  [v15 behaviour may differ] These commands do not yet use
           Shell-style argument quoting and therefore do not know about input tokens, so that
           syntax elements have to be surrounded by whitespace; in v15 S-nail will inspect all
           conditions bracket group wise and consider the tokens, representing values and
           operators, therein, which also means that variables will already have been expanded at
           that time (just like in the shell).

                 if receive
                   commands ...
                 else
                   commands ...
                 endif

           The (case-insensitive) condition ‘t’erminal will evaluate to true if the standard
           input is a terminal, i.e., in interactive sessions.  Another condition can be any
           boolean value (see the section INTERNAL VARIABLES for textual boolean representations)
           to mark an enwrapped block as “never execute” or “always execute”.  (It shall be
           remarked that a faulty condition skips all branches until endif.)

           ([v15 behaviour may differ] In v15 Shell-style argument quoting will be used, and this
           command will simply interpret expanded tokens.)  It is possible to check INTERNAL
           VARIABLES as well as ENVIRONMENT variables for existence or compare their expansion
           against a user given value or another variable by using the ‘$’ (“variable next”)
           conditional trigger character; a variable on the right hand side may be signalled
           using the same mechanism.  Variable names may be enclosed in a pair of matching
           braces.  When this mode has been triggered, several operators are available:

           Integer operators treat the arguments on the left and right hand side of the operator
           as integral numbers and compare them arithmetically.  It is an error if any of the
           operands is not a valid integer, an empty argument (which implies it had been quoted)
           is treated as if it were 0.  Available operators are ‘-lt’ (less than), ‘-le’ (less
           than or equal to), ‘-eq’ (equal), ‘-ne’ (not equal), ‘-ge’ (greater than or equal to),
           and ‘-gt’ (greater than).

           String data operators compare the left and right hand side according to their textual
           content.  Unset variables are treated as the empty string.  The behaviour of string
           operators can be adjusted by prefixing the operator with the modifier trigger
           commercial at ‘@’, followed by none to multiple modifiers: for now supported is ‘i’,
           which turns the comparison into a case-insensitive one: this is implied if no modifier
           follows the trigger.

           Available string operators are ‘<’ (less than), ‘<=’ (less than or equal to), ‘==’
           (equal), ‘!=’ (not equal), ‘>=’ (greater than or equal to), ‘>’ (greater than), ‘=%’
           (is substring of) and ‘!%’ (is not substring of).  By default these operators work on
           bytes and (therefore) do not take into account character set specifics.  If the case-
           insensitivity modifier has been used, case is ignored according to the rules of the
           US-ASCII encoding, i.e., bytes are still compared.

           When the [Option]al regular expression support is available, the additional string
           operators ‘=~’ and ‘!~’ can be used.  They treat the right hand side as an extended
           regular expression that is matched according to the active locale (see Character
           sets), i.e., character sets should be honoured correctly.

           Conditions can be joined via AND-OR lists (where the AND operator is ‘&&’ and the OR
           operator is ‘||’), which have equal precedence and will be evaluated with left
           associativity, thus using the same syntax that is known for the sh(1).  It is also
           possible to form groups of conditions and lists by enclosing them in pairs of brackets
           ‘[ ... ]’, which may be interlocked within each other, and also be joined via AND-OR
           lists.

           The results of individual conditions and entire groups may be modified via unary
           operators: the unary operator ‘!’ will reverse the result.

                 # (This not in v15, there [ -n "$debug"]!)
                 if $debug
                   echo *debug* is set
                 endif
                 if [ "$ttycharset" == UTF-8 ] || \
                     [ "$ttycharset" @i== UTF8 ]
                   echo *ttycharset* is UTF-8, the former case-sensitive!
                 endif
                 set t1=one t2=one
                 if [ "${t1}" == "${t2}" ]
                   echo These two variables are equal
                 endif
                 if [ "$features" =% +regex ] && \
                     [ "$TERM" @i=~ "^xterm.*" ]
                   echo ..in an X terminal
                 endif
                 if [ [ true ] && [ [ "${debug}" != '' ] || \
                     [ "$verbose" != '' ] ] ]
                   echo Noisy, noisy
                 endif
                 if true && [ "$debug" != '' ] || [ "${verbose}" != '' ]
                   echo Left associativity, as is known from the shell
                 endif

     ignore
           (ig) Identical to discard.  Superseded by the multiplexer headerpick.

     list  Shows the names of all available commands, alphabetically sorted.  If given any non-
           whitespace argument the list will be shown in the order in which command prefixes are
           searched.  [Option] In conjunction with a set variable verbose additional information
           will be provided for each command: the argument type will be indicated, the
           documentation string will be shown, and the set of command flags will show up:

           ‘`local'’    command supports the command modifier local.
           ‘`vput'’     command supports the command modifier vput.
           ‘*!*’        the error number is tracked in !.
           ‘needs-box’  whether the command needs an active mailbox, a file.
           ‘ok:’        indicators whether command is ...
                        ‘batch/interactive’
                                      usable in interactive or batch mode (-#).
                        ‘send-mode’   usable in send mode.
                        ‘subprocess’  allowed to be used when running in a subprocess instance,
                                      e.g., from within a macro that is called via
                                      on-compose-splice.
           ‘not ok:’    indicators whether command is not ...
                        ‘compose-mode’  available in compose mode.
                        ‘startup’       available during program startup, e.g., in Resource
                                        files.
           ‘gabby’      The command produces history-gabby history entries.

     localopts
           This command can be used to localize changes to (linked) ENVIRONMENT as well as
           (global) INTERNAL VARIABLES, meaning that their state will be reverted to the former
           one once the “covered scope” is left.  Just like the command modifier local, which
           provides block-scope localization for some commands (instead), it can only be used
           inside of macro definition blocks introduced by account or define.  The covered scope
           of an account is left once a different account is activated, and some macros, notably
           folder-hooks, use their own specific notion of covered scope, here it will be extended
           until the folder is left again.

           This setting stacks up: i.e., if ‘macro1’ enables change localization and calls
           ‘macro2’, which explicitly resets localization, then any value changes within ‘macro2’
           will still be reverted when the scope of ‘macro1’ is left.  (Caveats: if in this
           example ‘macro2’ changes to a different account which sets some variables that are
           already covered by localizations, their scope will be extended, and in fact leaving
           the account will (thus) restore settings in (likely) global scope which actually were
           defined in a local, macro private context!)

           This command takes one or two arguments, the optional first one specifies an attribute
           that may be one of scope, which refers to the current scope and is thus the default,
           call, which causes any macro that is being called to be started with localization
           enabled by default, as well as call-fixate, which (if enabled) disallows any called
           macro to turn off localization: like this it can be ensured that once the current
           scope regains control, any changes made in deeper levels have been reverted.  The
           latter two are mutually exclusive, and neither affects xcall.  The (second) argument
           is interpreted as a boolean (string, see INTERNAL VARIABLES) and states whether the
           given attribute shall be turned on or off.

                 define temporary_settings {
                   set possibly_global_option1
                   localopts on
                   set localized_option1
                   set localized_option2
                   localopts scope off
                   set possibly_global_option2
                 }

     Lreply
           Reply to messages that come in via known (mlist) or subscribed (mlsubscribe) mailing
           lists, or pretend to do so (see Mailing lists): on top of the usual reply
           functionality this will actively resort and even remove message recipients in order to
           generate a message that is supposed to be sent to a mailing list.  For example it will
           also implicitly generate a ‘Mail-Followup-To:’ header if that seems useful, regardless
           of the setting of the variable followup-to.  For more documentation please refer to On
           sending mail, and non-interactive mode.

           This may generate the errors ^ERR-DESTADDRREQ if no receiver has been specified,
           ^ERR-PERM if some addressees where rejected by expandaddr, ^ERR-NODATA if no
           applicable messages have been given, ^ERR-IO if an I/O error occurs, ^ERR-NOTSUP if a
           necessary character set conversion fails, and ^ERR-INVAL for other errors.  Occurrence
           of some of the errors depend on the value of expandaddr.  Any error stops processing
           of further messages.

     Mail  Similar to mail, but saves the message in a file named after the local part of the
           first recipient's address (instead of in record).

     mail  (m) Takes a (list of) recipient address(es) as (an) argument(s), or asks on standard
           input if none were given; then collects the remaining mail content and sends it out.
           Unless the internal variable fullnames is set recipient addresses will be stripped
           from comments, names etc.  For more documentation please refer to On sending mail, and
           non-interactive mode.

           This may generate the errors ^ERR-DESTADDRREQ if no receiver has been specified,
           ^ERR-PERM if some addressees where rejected by expandaddr, ^ERR-NODATA if no
           applicable messages have been given, ^ERR-NOTSUP if multiple messages have been
           specified, ^ERR-IO if an I/O error occurs, ^ERR-NOTSUP if a necessary character set
           conversion fails, and ^ERR-INVAL for other errors.  Occurrence of some of the errors
           depend on the value of expandaddr.

     mbox  (mb) The given message list is to be sent to the secondary mailbox MBOX when S-nail is
           quit; this is the default action unless the variable hold is set.  [v15 behaviour may
           differ] This command can only be used in a primary system mailbox.

     mimetype, unmimetype
           Without arguments the content of the MIME type cache will displayed; a more verbose
           listing will be produced if either of debug or verbose are set.  When given arguments
           they will be joined, interpreted as shown in The mime.types files (also see HTML mail
           and MIME attachments), and the resulting entry will be added (prepended) to the cache.
           In any event MIME type sources are loaded first as necessary – mimetypes-load-control
           can be used to fine-tune which sources are actually loaded.

           The latter command deletes all specifications of the given MIME type, thus ‘?
           unmimetype text/plain’ will remove all registered specifications for the MIME type
           ‘text/plain’.  The special name ‘*’ will discard all existing MIME types, just as will
           ‘reset’, but which also reenables cache initialization via mimetypes-load-control.

     mlist, unmlist
           The latter command removes all given mailing lists, the special name ‘*’ can be used
           to remove all registered lists.  The former will list all currently defined mailing
           lists (and their attributes, if any) when used without arguments; a more verbose
           listing will be produced if either of debug or verbose are set.  Otherwise all given
           arguments will be added and henceforth be recognized as mailing lists.  If the
           [Option]al regular expression support is available then any argument which contains
           any of the “magical” regular expression characters ‘^[]*+?|$’ (see re_format(7)) will
           be interpreted as one, which allows matching of many addresses with a single
           expression.  The mlsubscribe pair of commands manages subscription attributes of
           mailing lists.

     mimeview
           [v15 behaviour may differ] Only available in interactive mode, this command allows one
           to display MIME parts which require external MIME handler programs to run which do not
           integrate in S-nails normal type output (see HTML mail and MIME attachments).  ([v15
           behaviour may differ] No syntax to directly address parts, this restriction may
           vanish.)  The user will be asked for each non-text part of the given message in turn
           whether the registered handler shall be used to display the part.

     mlsubscribe, unmlsubscribe
           The latter command removes the subscription attribute from all given mailing lists,
           the special name ‘*’ can be used to do so for any registered list.  The former will
           list all currently defined mailing lists which have a subscription attribute when used
           without arguments; a more verbose listing will be produced if either of debug or
           verbose are set.  Otherwise this attribute will be set for all given mailing lists,
           newly creating them as necessary (as via mlist).  Also see followup-to.

     Move  Similar to move, but moves the messages to a file named after the local part of the
           sender address of the first message (instead of in record).

     move  Acts like copy but marks the messages for deletion if they were transferred
           successfully.

     More  Like more, but also displays header fields which would not pass the headerpick
           selection, and all MIME parts.  Identical to Page.

     more  Invokes the PAGER on the given messages, even in non-interactive mode and as long as
           the standard output is a terminal.  Identical to page.

     netrc
           [Option] When used without arguments or if show has been given the content of the ~/
           .netrc cache is shown, loading it first as necessary.  If the argument is load then
           the cache will only be initialized and clear will remove its contents.  Note that
           S-nail will try to load the file only once, use ‘netrc clear’ to unlock further
           attempts.  See netrc-lookup, netrc-pipe and the section On URL syntax and credential
           lookup; the section The .netrc file documents the file format in detail.

     newmail
           Checks for new mail in the current folder without committing any changes before.  If
           new mail is present, a message is shown.  If the header variable is set, the headers
           of each new message are also shown.  This command is not available for all mailbox
           types.

     next  (n) (like ‘+’ or “ENTER”) Goes to the next message in sequence and types it.  With an
           argument list, types the next matching message.

     New   Same as Unread.

     new   Same as unread.

     noop  If the current folder is accessed via a network connection, a “NOOP” command is sent,
           otherwise no operation is performed.

     Page  Like page, but also displays header fields which would not pass the headerpick
           selection, and all MIME parts.  Identical to More.

     page  Invokes the PAGER on the given messages, even in non-interactive mode and as long as
           the standard output is a terminal.  Identical to more.

     Pipe  Like pipe but also pipes header fields which would not pass the headerpick selection,
           and all parts of MIME ‘multipart/alternative’ messages.

     pipe  (pi) Takes an optional message list and shell command (that defaults to cmd), and
           pipes the messages through the command.  If the page variable is set, every message is
           followed by a formfeed character.

     preserve
           (pre) A synonym for hold.

     Print
           (P) Alias for Type.

     print
           (p) Research UNIX equivalent of type.

     quit  (q) Terminates the session, saving all undeleted, unsaved messages in the current
           secondary mailbox MBOX, preserving all messages marked with hold or preserve or never
           referenced in the system inbox, and removing all other messages from the primary
           system mailbox.  If new mail has arrived during the session, the message “You have new
           mail” will be shown.  If given while editing a mailbox file with the command line
           option -f, then the edit file is rewritten.  A return to the shell is effected, unless
           the rewrite of edit file fails, in which case the user can escape with the exit
           command.  The optional status number argument will be passed through to exit(3).  [v15
           behaviour may differ] For now it can happen that the given status will be overwritten,
           later this will only occur if a later error needs to be reported onto an otherwise
           success indicating status.

     read  [Only new quoting rules] Read a line from standard input, or the channel set active
           via readctl, and assign the data, which will be split as indicated by ifs, to the
           given variables.  The variable names are checked by the same rules as documented for
           vput, and the same error codes will be seen in !; the exit status ? indicates the
           number of bytes read, it will be ‘-1’ with the error number ! set to ^ERR-BADF in case
           of I/O errors, or ^ERR-NONE upon End-Of-File.  If there are more fields than
           variables, assigns successive fields to the last given variable.  If there are less
           fields than variables, assigns the empty string to the remains.

                 ? read a b c
                    H  e  l  l  o
                 ? echo "<$a> <$b> <$c>"
                 <H> <e> <l  l  o>
                 ? wysh set ifs=:; read a b c;unset ifs
                 hey2.0,:"'you    ",:world!:mars.:
                 ? echo $?/$^ERRNAME / <$a><$b><$c>
                 0/NONE / <hey2.0,><"'you    ",><world!:mars.:><><>

     readall
           [Only new quoting rules] Read anything from standard input, or the channel set active
           via readctl, and assign the data to the given variable.  The variable name is checked
           by the same rules as documented for vput, and the same error codes will be seen in !;
           the exit status ? indicates the number of bytes read, it will be ‘-1’ with the error
           number ! set to ^ERR-BADF in case of I/O errors, or ^ERR-NONE upon End-Of-File.  [v15
           behaviour may differ] The input data length is restricted to 31-bits.

     readctl
           [Only new quoting rules] Manages input channels for read and readall, to be used to
           avoid complicated or impracticable code, like calling read from within a macro in non-
           interactive mode.  Without arguments, or when the first argument is show, a listing of
           all known channels is printed.  Channels can otherwise be created, and existing
           channels can be set active and removed by giving the string used for creation.

           The channel name is expected to be a file descriptor number, or, if parsing the
           numeric fails, an input file name that undergoes Filename transformations.  E.g. (this
           example requires a modern shell):

                 $ LC_ALL=C printf 'echon "hey, "\nread a\nyou\necho $a' |\
                   LC_ALL=C s-nail -R#
                 hey, you
                 $ LC_ALL=C printf 'echon "hey, "\nread a\necho $a' |\
                   LC_ALL=C 6<<< 'you' s-nail -R#X'readctl create 6'
                 hey, you

     remove
           Removes the named files or directories.  Filename transformations including shell
           pathname wildcard pattern expansions (glob(7)) are performed on the arguments.  If a
           name refer to a mailbox, e.g., a Maildir mailbox, then a mailbox type specific removal
           will be performed, deleting the complete mailbox.  The user is asked for confirmation
           in interactive mode.

     rename
           Takes the name of an existing folder and the name for the new folder and renames the
           first to the second one.  Filename transformations including shell pathname wildcard
           pattern expansions (glob(7)) are performed on both arguments.  Both folders must be of
           the same type.

     Reply
           (R) Identical to reply except that it replies to only the sender of each message of
           the given list, by using the first message as the template to quote, for the
           ‘Subject:’ etc.; setting flipr will exchange this command with reply.

     reply
           (r) Take a message and group-responds to it by addressing the sender and all
           recipients, subject to alternates processing.  followup-to, followup-to-honour,
           reply-to-honour as well as recipients-in-cc influence response behaviour.  Unless the
           internal variable fullnames is set recipient addresses will be stripped from comments,
           names etc.  quote as well as quote-as-attachment configure whether responded-to
           message shall be quoted etc.; setting flipr will exchange this command with Reply.
           The command Lreply offers special support for replying to mailing lists.  For more
           documentation please refer to On sending mail, and non-interactive mode.

           This may generate the errors ^ERR-DESTADDRREQ if no receiver has been specified,
           ^ERR-PERM if some addressees where rejected by expandaddr, ^ERR-NODATA if no
           applicable messages have been given, ^ERR-IO if an I/O error occurs, ^ERR-NOTSUP if a
           necessary character set conversion fails, and ^ERR-INVAL for other errors.  Occurrence
           of some of the errors depend on the value of expandaddr.  Any error stops processing
           of further messages.

     replyall
           Similar to reply, but initiates a group-reply regardless of the value of flipr.

     replysender
           Similar to Reply, but responds to the sender only regardless of the value of flipr.

     Resend
           Like resend, but does not add any header lines.  This is not a way to hide the
           sender's identity, but useful for sending a message again to the same recipients.

     resend
           Takes a list of messages and a user name and sends each message to the named user.
           ‘Resent-From:’ and related header fields are prepended to the new copy of the message.
           Saving in record is only performed if record-resent is set.

           This may generate the errors ^ERR-DESTADDRREQ if no receiver has been specified,
           ^ERR-PERM if some addressees where rejected by expandaddr, ^ERR-NODATA if no
           applicable messages have been given, ^ERR-IO if an I/O error occurs, ^ERR-NOTSUP if a
           necessary character set conversion fails, and ^ERR-INVAL for other errors.  Occurrence
           of some of the errors depend on the value of expandaddr.  Any error stops processing
           of further messages.

     Respond
           Same as Reply.

     respond
           Same as reply.

     respondall
           Same as replyall.

     respondsender
           Same as replysender.

     retain
           (ret) Superseded by the multiplexer headerpick.

     return
           Only available inside the scope of a defined macro or an account, this will stop
           evaluation of any further macro content, and return execution control to the caller.
           The two optional parameters must be specified as positive decimal numbers and default
           to the value 0: the first argument specifies the signed 32-bit return value (stored in
           ? [v15 behaviour may differ] and later extended to signed 64-bit), the second the
           signed 32-bit error number (stored in !).  As documented for ? a non-0 exit status may
           cause the program to exit.

     Save  (S) Similar to save, but saves the messages in a file named after the local part of
           the sender of the first message instead of (in record and) taking a filename argument;
           the variable outfolder is inspected to decide on the actual storage location.

     save  (s) Takes a message list and a filename and appends each message in turn to the end of
           the file.  Filename transformations including shell pathname wildcard pattern
           expansions (glob(7)) is performed on the filename.  If no filename is given, the
           secondary mailbox MBOX is used.  The filename in quotes, followed by the generated
           character count is echoed on the user's terminal.  If editing a primary system mailbox
           the messages are marked for deletion.  Filename transformations will be applied.  To
           filter the saved header fields to the desired subset use the ‘save’ slot of the white-
           and blacklisting command headerpick.

     savediscard
           [Obsolete] Superseded by the multiplexer headerpick.

     saveignore
           [Obsolete] Superseded by the multiplexer headerpick.

     saveretain
           [Obsolete] Superseded by the multiplexer headerpick.

     search
           Takes a message specification (list) and displays a header summary of all matching
           messages, as via headers.  This command is an alias of from.  Also see Specifying
           messages.

     seen  Takes a message list and marks all messages as having been read.

     set, unset
           (se, [Only new quoting rules] uns) The latter command will delete all given global
           variables, or only block-scope local ones if the local command modifier has been used.
           The former, when used without arguments, will show all currently known variables,
           being more verbose if either of debug or verbose is set.  Remarks: this list mode will
           not automatically link-in known ENVIRONMENT variables, but only explicit addressing
           will, e.g., via varshow, using a variable in an if condition or a string passed to
           echo, explicit setting, as well as some program-internal use cases.

           Otherwise the given variables (and arguments) are set or adjusted.  Arguments are of
           the form ‘name=value’ (no space before or after ‘=’), or plain ‘name’ if there is no
           value, i.e., a boolean variable.  If a name begins with ‘no’, as in ‘set nosave’, the
           effect is the same as invoking the unset command with the remaining part of the
           variable (‘unset save’).  [v15 behaviour may differ] In conjunction with the wysh
           (or local) command prefix(es) Shell-style argument quoting can be used to quote
           arguments as necessary.  [v15 behaviour may differ] Otherwise quotation marks may be
           placed around any part of the assignment statement to quote blanks or tabs.

           When operating in global scope any ‘name’ that is known to map to an environment
           variable will automatically cause updates in the program environment (unsetting a
           variable in the environment requires corresponding system support) — use the command
           environ for further environmental control.  If the command modifier local has been
           used to alter the command to work in block-scope all variables have values (may they
           be empty), and creation of names which shadow INTERNAL VARIABLES is actively prevented
           ([v15 behaviour may differ] shadowing of linked ENVIRONMENT variables and free-form
           versions of variable chains is not yet detected).  Also see varshow and the sections
           INTERNAL VARIABLES and ENVIRONMENT.

                 ? wysh set indentprefix=' -> '
                 ? wysh set atab=$'' aspace=' ' zero=0

     shcodec
           Apply shell quoting rules to the given raw-data arguments.  Supports vput (see Command
           modifiers).  The first argument specifies the operation: [+]e[ncode] or d[ecode] cause
           shell quoting to be applied to the remains of the line, and expanded away thereof,
           respectively.  If the former is prefixed with a plus-sign, the quoted result will not
           be roundtrip enabled, and thus can be decoded only in the very same environment that
           was used to perform the encode; also see mle-quote-rndtrip.  If the coding operation
           fails the error number ! is set to ^ERR-CANCELED, and the unmodified input is used as
           the result; the error number may change again due to output or result storage errors.

     shell
           [Only new quoting rules] (sh) Invokes an interactive version of the shell, and returns
           its exit status.

     shortcut, unshortcut
           Without arguments the list of all currently defined shortcuts is shown, with one
           argument the expansion of the given shortcut.  Otherwise all given arguments are
           treated as pairs of shortcuts and their expansions, creating new or changing already
           existing shortcuts, as necessary.  The latter command will remove all given shortcuts,
           the special name ‘*’ will remove all registered shortcuts.

     shift
           [Only new quoting rules] Shift the positional parameter stack (starting at 1) by the
           given number (which must be a positive decimal), or 1 if no argument has been given.
           It is an error if the value exceeds the number of positional parameters.  If the given
           number is 0, no action is performed, successfully.  The stack as such can be managed
           via vpospar.  Note this command will fail in account and hook macros unless the
           positional parameter stack has been explicitly created in the current context via
           vpospar.

     show  Like type, but performs neither MIME decoding nor decryption, so that the raw message
           text is shown.

     size  (si) Shows the size in characters of each message of the given message-list.

     sleep
           [Only new quoting rules] Sleep for the specified number of seconds (and optionally
           milliseconds), by default interruptably.  If a third argument is given the sleep will
           be uninterruptible, otherwise the error number ! will be set to ^ERR-INTR if the sleep
           has been interrupted.  The command will fail and the error number will be
           ^ERR-OVERFLOW if the given duration(s) overflow the time datatype, and ^ERR-INVAL if
           the given durations are no valid integers.

     sort, unsort
           The latter command disables sorted or threaded mode, returns to normal message order
           and, if the header variable is set, displays a header summary.  The former command
           shows the current sorting criterion when used without an argument, but creates a
           sorted representation of the current folder otherwise, and changes the next command
           and the addressing modes such that they refer to messages in the sorted order.
           Message numbers are the same as in regular mode.  If the header variable is set, a
           header summary in the new order is also displayed.  Automatic folder sorting can be
           enabled by setting the autosort variable, as in, e.g., ‘set autosort=thread’.
           Possible sorting criterions are:

           date     Sort the messages by their ‘Date:’ field, that is by the time they were sent.
           from     Sort messages by the value of their ‘From:’ field, that is by the address of
                    the sender.  If the showname variable is set, the sender's real name (if any)
                    is used.
           size     Sort the messages by their size.
           spam     [Option] Sort the message by their spam score, as has been classified by
                    spamrate.
           status   Sort the messages by their message status.
           subject  Sort the messages by their subject.
           thread   Create a threaded display.
           to       Sort messages by the value of their ‘To:’ field, that is by the address of
                    the recipient.  If the showname variable is set, the recipient's real name
                    (if any) is used.

     source
           [Only new quoting rules] (so) The source command reads commands from the given file.
           Filename transformations will be applied.  If the given expanded argument ends with a
           vertical bar ‘|’ then the argument will instead be interpreted as a shell command and
           S-nail will read the output generated by it.  Dependent on the settings of posix and
           errexit, and also dependent on whether the command modifier ignerr had been used,
           encountering errors will stop sourcing of the given input.  [v15 behaviour may differ]
           Note that source cannot be used from within macros that execute as folder-hooks or
           accounts, i.e., it can only be called from macros that were called.

     source_if
           [Only new quoting rules] The difference to source (beside not supporting pipe syntax
           aka shell command input) is that this command will not generate an error nor warn if
           the given file argument cannot be opened successfully.

     spamclear
           [Option] Takes a list of messages and clears their ‘is-spam’ flag.

     spamforget
           [Option] Takes a list of messages and causes the spam-interface to forget it has ever
           used them to train its Bayesian filter.  Unless otherwise noted the ‘is-spam’ flag of
           the message is inspected to chose whether a message shall be forgotten to be “ham” or
           “spam”.

     spamham
           [Option] Takes a list of messages and informs the Bayesian filter of the
           spam-interface that they are “ham”.  This also clears the ‘is-spam’ flag of the
           messages in question.

     spamrate
           [Option] Takes a list of messages and rates them using the configured spam-interface,
           without modifying the messages, but setting their ‘is-spam’ flag as appropriate;
           because the spam rating headers are lost the rate will be forgotten once the mailbox
           is left.  Refer to the manual section Handling spam for the complete picture of spam
           handling in S-nail.

     spamset
           [Option] Takes a list of messages and sets their ‘is-spam’ flag.

     spamspam
           [Option] Takes a list of messages and informs the Bayesian filter of the
           spam-interface that they are “spam”.  This also sets the ‘is-spam’ flag of the
           messages in question.

     thread
           [Obsolete] The same as ‘sort thread’ (consider using a ‘commandalias’ as necessary).

     tls   [Only new quoting rules] TLS information and management command multiplexer to aid in
           Encrypted network communication.  Commands support vput if so documented (see Command
           modifiers).  The result that is shown in case of errors is always the empty string,
           errors can be identified via the error number !.  For example, string length overflows
           are caught and set ! to ^ERR-OVERFLOW.  Note this command of course honours the
           overall TLS configuration.

                 ? vput tls result fingerprint pop3s://ex.am.ple
                 ? echo $?/$!/$^ERRNAME: $result

           fingerprint Show the tls-fingerprint-digested fingerprint of the certificate of the
                     given HOST (‘server:port’, where the port defaults to the HTTPS port, 443).
                     tls-fingerprint is actively ignored for the runtime of this command.  Only
                     available if the term ‘+sockets’ is included in features.

     Top   Like top but always uses the headerpick ‘type’ slot for white- and blacklisting header
           fields.

     top   (to) Takes a message list and types out the first toplines lines of each message on
           the user's terminal.  Unless a special selection has been established for the ‘top’
           slot of the headerpick command, the only header fields that are displayed are ‘From:’,
           ‘To:’, ‘CC:’, and ‘Subject:’.  Top will always use the ‘type’ headerpick selection
           instead.  It is possible to apply compression to what is displayed by setting
           topsqueeze.  Messages are decrypted and converted to the terminal character set if
           necessary.

     touch
           (tou) Takes a message list and marks the messages for saving in the secondary mailbox
           MBOX.  S-nail deviates from the POSIX standard with this command, as a following next
           command will display the following message instead of the current one.

     Type  (T) Like type but also displays header fields which would not pass the headerpick
           selection, and all visualizable parts of MIME ‘multipart/alternative’ messages.

     type  (t) Takes a message list and types out each message on the user's terminal.  The
           display of message headers is selectable via headerpick.  For MIME multipart messages,
           all parts with a content type of ‘text’, all parts which have a registered MIME type
           handler (see HTML mail and MIME attachments) which produces plain text output, and all
           ‘message’ parts are shown, others are hidden except for their headers.  Messages are
           decrypted and converted to the terminal character set if necessary.  The command
           mimeview can be used to display parts which are not displayable as plain text.

     unaccount
           See account.

     unalias
           (una) See alias.

     unanswered
           See answered.

     unbind
           See bind.

     uncollapse
           See collapse.

     uncolour
           See colour.

     undefine
           See define.

     undelete
           See delete.

     undraft
           See draft.

     unflag
           See flag.

     unfwdignore
           [Obsolete] Superseded by the multiplexer headerpick.

     unfwdretain
           [Obsolete] Superseded by the multiplexer headerpick.

     unignore
           Superseded by the multiplexer headerpick.

     unmimetype
           See mimetype.

     unmlist
           See mlist.

     unmlsubscribe
           See mlsubscribe.

     Unread
           Same as unread.

     unread
           Takes a message list and marks each message as not having been read.

     unretain
           Superseded by the multiplexer headerpick.

     unsaveignore
           [Obsolete] Superseded by the multiplexer headerpick.

     unsaveretain
           [Obsolete] Superseded by the multiplexer headerpick.

     unset
           [Only new quoting rules] (uns) See set.

     unshortcut
           See shortcut.

     unsort
           See short.

     unthread
           [Obsolete] Same as unsort.

     urlcodec
           Perform URL percent codec operations on the raw-data argument, rather according to RFC
           3986.  Supports vput (see Command modifiers), and manages the error number !.  This is
           a character set agnostic and thus locale dependent operation, and it may decode bytes
           which are invalid in the current ttycharset.  [v15 behaviour may differ] This command
           does not know about URLs beside that.

           The first argument specifies the operation: e[ncode] or d[ecode] perform plain URL
           percent en- and decoding, respectively.  p[ath]enc[ode] and p[ath]dec[ode] perform a
           slightly modified operation which should be better for pathnames: it does not allow a
           tilde ‘~’, and will neither accept hyphen-minus ‘-’ nor dot ‘’.  as an initial
           character.  The remains of the line form the URL data which is to be converted.  If
           the coding operation fails the error number ! is set to ^ERR-CANCELED, and the
           unmodified input is used as the result; the error number may change again due to
           output or result storage errors.

     varshow
           [Only new quoting rules] This command produces the same output as the listing mode of
           set, including verboseity adjustments, but only for the given variables.

     verify
           [Option] Takes a message list and verifies each message.  If a message is not a S/MIME
           signed message, verification will fail for it.  The verification process checks if the
           message was signed using a valid certificate, if the message sender's email address
           matches one of those contained within the certificate, and if the message content has
           been altered.

     version
           Shows the version and features of S-nail, as well as the build and running system
           environment.  This command can produce a more verbose output, and supports vput (see
           Command modifiers).

     vexpr
           [Only new quoting rules] Evaluate arguments according to a given operator.  This is a
           multiplexer command which can be used to perform signed 64-bit numeric calculations as
           well as byte string and string operations.  It uses polish notation, i.e., the
           operator is the first argument and defines the number and type, and the meaning of the
           remaining arguments.  An empty argument is replaced with a 0 if a number is expected.
           Supports vput (see Command modifiers).

           The result that is shown in case of errors is always ‘-1’ for usage errors and numeric
           operations, and the empty string for byte string and string operations; if the latter
           two fail to provide result data for “soft” errors, e.g., when a search operation
           failed, they also set the ! error number to ^ERR-NODATA.  Except when otherwise noted
           numeric arguments are parsed as signed 64-bit numbers, and errors will be reported in
           the error number ! as the numeric error ^ERR-RANGE.

           Numeric operations work on one or two signed 64-bit integers.  Numbers prefixed with
           ‘0x’ or ‘0X’ are interpreted as hexadecimal (base 16) numbers, whereas ‘0’ indicates
           octal (base 8), and ‘0b’ as well as ‘0B’ denote binary (base 2) numbers.  It is
           possible to use any base in between 2 and 36, inclusive, with the ‘BASE#number’
           notation, where the base is given as an unsigned decimal number, e.g., ‘16#AFFE’ is a
           different way of specifying a hexadecimal number.  Unsigned interpretation of a number
           can be enforced by prefixing an ‘u’ (case-insensitively), e.g., ‘u-110’; this is not
           necessary for power-of-two bases (2, 4, 8, 16 and 32), which will be interpreted as
           unsigned by default, but it still makes a difference regarding overflow detection and
           overflow constant.  It is possible to enforce signed interpretation by (instead)
           prefixing a ‘s’ (case-insensitively).

           One integer is expected by assignment (equals sign ‘=’), which does nothing but
           parsing the argument, thus detecting validity and possible overflow conditions, and
           unary not (tilde ‘~’), which creates the bitwise complement.  Two integers are used by
           addition (plus sign ‘+’), subtraction (hyphen-minus ‘-’), multiplication (asterisk
           ‘*’), division (solidus ‘/’) and modulo (percent sign ‘%’), as well as for the bitwise
           operators logical or (vertical bar ‘|’, to be quoted) , bitwise and (ampersand ‘&’, to
           be quoted) , bitwise xor (circumflex ‘^’), the bitwise signed left- and right shifts
           (‘<<’, ‘>>’), as well as for the unsigned right shift ‘>>>’.

           Another numeric operation is pbase, which takes a number base in between 2 and 36,
           inclusive, and will act on the second number given just the same as what equals sign
           ‘=’ does, but the number result will be formatted in the base given.

           All numeric operators can be prefixed with a commercial at ‘@’, e.g., ‘@*’: this will
           turn the operation into a saturated one, which means that overflow errors and division
           and modulo by zero are no longer reported via the exit status, but the result will
           linger at the minimum or maximum possible value, instead of overflowing (or trapping).
           This is true also for the argument parse step.  For the bitwise shifts, the saturated
           maximum is 63.  Any caught overflow will be reported via the error number ! as
           ^ERR-OVERFLOW.

                 ? vexpr @- +1 -9223372036854775808
                 ? echo $?/$!/$^ERRNAME

           Character set agnostic string functions have no notion of locale settings and
           character sets.

           file-expand Performs the usual Filename transformations on its argument.

           random    Generates a random string of the given length, or of PATH_MAX bytes (a
                     constant from /usr/include) if the value 0 is given; the random string will
                     be base64url encoded according to RFC 4648, and thus be usable as a
                     (portable) filename.

           Byte string operations work on 8-bit bytes and have no notion of locale settings and
           character sets, effectively assuming ASCII data.

           length    Queries the length of the given argument.

           hash      Calculates the Chris Torek hash of the given argument.

           find      Byte-searches in the first for the second argument.  Shows the resulting
                     0-based offset shall it have been found.

           ifind     Identical to find, but works case-insensitively according to the rules of
                     the ASCII character set.

           substring Creates a substring of its first argument.  The second argument is the
                     0-based starting offset, a negative one counts from the end; the optional
                     third argument specifies the length of the desired result, a negative length
                     leaves off the given number of bytes at the end of the original string, by
                     default the entire string is used; this operation tries to work around
                     faulty arguments (set verbose for error logs), but reports them via the
                     error number ! as ^ERR-OVERFLOW.

           trim      Trim away whitespace characters from both ends of the argument.

           trim-front Trim away whitespace characters from the begin of the argument.

           trim-end  Trim away whitespace characters from the end of the argument.

           String operations work, sufficient support provided, according to the active user's
           locale encoding and character set (see Character sets).

           makeprint (One-way) Converts the argument to something safely printable on the
                     terminal.

           regex     [Option] A string operation that will try to match the first argument with
                     the regular expression given as the second argument.  If the optional third
                     argument has been given then instead of showing the match offset a
                     replacement operation is performed: the third argument is treated as if
                     specified within dollar-single-quote (see Shell-style argument quoting), and
                     any occurrence of a positional parameter, e.g., 1, is replaced by the
                     corresponding match group of the regular expression:

                           ? vput vexpr res regex bananarama \
                               (.*)NanA(.*) '\${1}au\$2'
                           ? echo $?/$!/$^ERRNAME: $res

           iregex    On otherwise identical case-insensitive equivalent to regex:

                           ? vput vexpr res ire bananarama \
                               (.*)NanA(.*) '\${1}au\$2'
                           ? echo $?/$!/$^ERRNAME: $res

     vpospar
           [Only new quoting rules] Manage the positional parameter stack (see 1, #, *, @ as well
           as shift).  If the first argument is ‘clear’, then the positional parameter stack of
           the current context, or the global one, if there is none, is cleared.  If it is ‘set’,
           then the remaining arguments will be used to (re)create the stack, if the parameter
           stack size limit is excessed an ^ERR-OVERFLOW error will occur.

           If the first argument is ‘quote’, a round-trip capable representation of the stack
           contents is created, with each quoted parameter separated from each other with the
           first character of ifs, and followed by the first character of if-ws, if that is not
           empty and not identical to the first.  If that results in no separation at all a space
           character is used.  This mode supports vput (see Command modifiers).  I.e., the
           subcommands ‘set’ and ‘quote’ can be used (in conjunction with eval) to (re)create an
           argument stack from and to a single variable losslessly.

                 ? vpospar set hey, "'you    ", world!
                 ? echo $#: <${1}><${2}><${3}>
                 ? vput vpospar x quote
                 ? vpospar clear
                 ? echo $#: <${1}><${2}><${3}>
                 ? eval vpospar set ${x}
                 ? echo $#: <${1}><${2}><${3}>

     visual
           (v) Takes a message list and invokes the VISUAL display editor on each message.
           Modified contents are discarded unless the writebackedited variable is set, and are
           not used unless the mailbox can be written to and the editor returns a successful exit
           status.  edit can be used instead for a less display oriented editor.

     write
           (w) For conventional messages the body without all headers is written.  The original
           message is never marked for deletion in the originating mail folder.  The output is
           decrypted and converted to its native format as necessary.  If the output file exists,
           the text is appended.  If a message is in MIME multipart format its first part is
           written to the specified file as for conventional messages, handling of the remains
           depends on the execution mode.  No special handling of compressed files is performed.

           In interactive mode the user is consecutively asked for the filenames of the processed
           parts.  For convience saving of each part may be skipped by giving an empty value, the
           same result as writing it to /dev/null.  Shell piping the part content by specifying a
           leading vertical bar ‘|’ character for the filename is supported.  Other user input
           undergoes the usual Filename transformations, including shell pathname wildcard
           pattern expansions (glob(7)) and shell variable expansion for the message as such, not
           the individual parts, and contents of the destination file are overwritten if the file
           previously existed.

           [v15 behaviour may differ] In non-interactive mode any part which does not specify a
           filename is ignored, and suspicious parts of filenames of the remaining parts are URL
           percent encoded (as via urlcodec) to prevent injection of malicious character
           sequences, resulting in a filename that will be written into the current directory.
           Existing files will not be overwritten, instead the part number or a dot are appended
           after a number sign ‘#’ to the name until file creation succeeds (or fails due to
           other reasons).

     xcall
           [Only new quoting rules] The sole difference to call is that the new macro is executed
           in place of the current one, which will not regain control: all resources of the
           current macro will be released first.  This implies that any setting covered by
           localopts will be forgotten and covered variables will become cleaned up.  If this
           command is not used from within a called macro it will silently be (a more expensive
           variant of) call.

     xit   (x) A synonym for exit.

     z     [Only new quoting rules] S-nail presents message headers in screenfuls as described
           under the headers command.  Without arguments this command scrolls to the next window
           of messages, likewise if the argument is ‘+’.  An argument of ‘-’ scrolls to the last,
           ‘^’ scrolls to the first, and ‘$’ to the last screen of messages.  A number argument
           prefixed by ‘+’ or ‘-’ indicates that the window is calculated in relation to the
           current position, and a number without a prefix specifies an absolute position.

     Z     [Only new quoting rules] Similar to z, but scrolls to the next or previous window that
           contains at least one ‘new’ or flagged message.

COMMAND ESCAPES

     Command escapes are available in compose mode, and are used to perform special functions
     when composing messages.  Command escapes are only recognized at the beginning of lines, and
     consist of a trigger (escape), and a command character.  The actual escape character can be
     set via the internal variable escape, it defaults to the tilde ‘~’.  Otherwise ignored
     whitespace characters following the escape character will prevent a possible addition of the
     command line to the [Option]al history.

     Unless otherwise noted all compose mode command escapes ensure proper updates of the
     variables which represent the error number ! and the exit status ?.  If the variable errexit
     is set they will, unless stated otherwise, error out message compose mode and cause a
     program exit if an operation fails; an effect equivalent to the command modifier ignerr can
     however be achieved by placing a hyphen-minus ‘-’ after (possible whitespace following) the
     escape character.  If the [Option]al key bindings are available it is possible to create
     bindings specifically for the compose mode.

     ~~ string
           Insert the string of text in the message prefaced by a single ‘~’.  (If the escape
           character has been changed, that character must be doubled instead.)

     ~! command
           Execute the indicated shell command which follows, replacing unescaped exclamation
           marks with the previously executed command if the internal variable bang is set, then
           return to the message.

     ~.    End compose mode and send the message.  The hooks on-compose-splice-shell and
           on-compose-splice, in order, will be called when set, after which askatend will be
           checked, a set on-compose-leave hook will be called, autocc and autobcc will be joined
           in if set, asksend will be honoured in interactive mode, finally a given
           message-inject-tail will be incorporated, after which the compose mode is left.

     ~: S-nail-command or ~_ S-nail-command
           Execute the given S-nail command.  Not all commands, however, are allowed.

     ~< filename
           Identical to ~r.

     ~<! command
           command is executed using the shell.  Its standard output is inserted into the
           message.

     ~?    [Option] Write a summary of command escapes.

     ~@ [filename...]
           Append or edit the list of attachments.  Does not manage the error number ! and the
           exit status ? (please use ~^ instead if this is a concern).  The append mode expects a
           list of filename arguments as shell tokens (see Shell-style argument quoting; token-
           separating commas are ignored, too), to be interpreted as documented for the command
           line option -a, with the message number exception as below.

           Without filename arguments the attachment list is edited, entry by entry; if a
           filename is left empty, that attachment is deleted from the list; once the end of the
           list is reached either new attachments may be entered or the session can be quit by
           committing an empty “new” attachment.  In non-interactive mode or in batch mode (-#)
           the list of attachments is effectively not edited but instead recreated; again, an
           empty input ends list creation.

           For all modes, if a given filename solely consists of the number sign ‘#’ followed by
           either a valid message number of the currently active mailbox, or by a period ‘.’,
           referring to the current message of the active mailbox, the so-called “dot”, then the
           given message is attached as a ‘message/rfc822’ MIME message part.  The number sign
           must be quoted to avoid misinterpretation with the shell comment character.

     ~| command
           Pipe the message text through the specified filter command.  If the command gives no
           output or terminates abnormally, retain the original text of the message.  E.g., the
           command fmt(1) is often used as a rejustifying filter.

           If the first character of the command is a vertical bar, then the entire message
           including header fields is subject to the filter command, e.g., ‘~|| echo Fcc:
           /tmp/test; cat’ will prepend a file-carbon-copy message header.  Also see ~e, ~v.

     ~^ cmd [subcmd [arg3 [arg4]]]
           Low-level compose mode command which shares the semantic with digmsg and is meant for
           scripted message access, i.e., for on-compose-splice and on-compose-splice-shell.  The
           used protocol is likely subject to changes, and therefore the mentioned hooks receive
           the used protocol version as an initial line.  In general the first field of a
           response line represents a status code which specifies whether a command was
           successful or not, whether result data is to be expected, and if, the format of the
           result data.  Does not manage the error number ! and the exit status ?, because errors
           are reported via the protocol (hard errors like I/O failures cannot be handled).

           This command has read-only access to several virtual pseudo headers in the S-nail
           private namespace which optionally (except for the non-optional first) exist in
           compose mode:

           ‘Mailx-Command:’
                 The name of the command that generates the message, one of ‘forward’, ‘Lreply’,
                 ‘mail’, ‘Reply’, ‘reply’, ‘resend’.
           ‘Mailx-Raw-To:’
           ‘Mailx-Raw-Cc:’
           ‘Mailx-Raw-Bcc:’
                 Represent the frozen initial state of these headers before any transformation
                 (e.g., alias, alternates, recipients-in-cc etc.) took place.
           ‘Mailx-Orig-From:’
           ‘Mailx-Orig-To:’
           ‘Mailx-Orig-Cc:’
           ‘Mailx-Orig-Bcc:’
                 The values of said headers of the original message which has been addressed by
                 any of reply, forward, resend.

           Error status code lines may optionally contain additional context.  The status codes
           are:

           ‘210’  Status ok; the remains of the line are the result.
           ‘211’  Status ok; the rest of the line is optionally used for more status.  What
                  follows are lines of result addresses, terminated by an empty line.  The
                  address lines consist of two fields, the first of which is the plain address,
                  e.g., ‘bob@exam.ple’, separated by a single ASCII SP space from the second
                  which contains the unstripped address, even if that is identical to the first
                  field, e.g., ‘(Lovely) Bob <bob@exam.ple>’.  Non-network addressees, however,
                  place a single-letter indicating the address type in the first field (hyphen-
                  minus ‘-’ for files, vertical bar ‘|’ for pipes, and number sign ‘#’ for names:
                  what is supposed to become expanded via alias), and only the second field
                  contains a value.  All the input, including the empty line, must be consumed
                  before further commands can be issued.
           ‘212’  Status ok; the rest of the line is optionally used for more status.  What
                  follows are lines of furtherly unspecified string content, terminated by an
                  empty line.  All the input, including the empty line, must be consumed before
                  further commands can be issued.
           ‘500’  Syntax error; invalid command.
           ‘501’  Syntax error in parameters or arguments.
           ‘505’  Error: an argument fails verification.  For example an invalid address has been
                  specified (also see expandaddr), or an attempt was made to modify anything in
                  S-nail's own namespace, or a modifying subcommand has been used on a read-only
                  message.
           ‘506’  Error: an otherwise valid argument is rendered invalid due to context.  For
                  example, a second address is added to a header which may consist of a single
                  address only.

           If a command indicates failure then the message will have remained unmodified.  Most
           commands can fail with ‘500’ if required arguments are missing (false command usage).
           The following (case-insensitive) commands are supported:

           version  This command will print the protocol version via 210.

           header   This command allows listing, inspection, and editing of message headers.
                    Header name case is not normalized, and case-insensitive comparison should be
                    used when matching names.  The second argument specifies the subcommand to
                    apply, one of:

                    list      Without a third argument a list of all yet existing headers is
                              given via ‘210’; this command is the default command of header if
                              no second argument has been given.  A third argument restricts
                              output to the given header only, which may fail with ‘501’ if no
                              such field is defined.

                    show      Shows the content of the header given as the third argument.
                              Dependent on the header type this may respond with ‘211’ or ‘212’;
                              any failure results in ‘501’.

                    remove    This will remove all instances of the header given as the third
                              argument, reporting ‘210’ upon success, ‘501’ if no such header can
                              be found, and Ql 505 on S-nail namespace violations.

                    remove-at This will remove from the header given as the third argument the
                              instance at the list position (counting from one!) given with the
                              fourth argument, reporting ‘210’ upon success or ‘505’ if the list
                              position argument is not a number or on S-nail namespace
                              violations, and ‘501’ if no such header instance exists.

                    insert    Create a new or an additional instance of the header given in the
                              third argument, with the header body content as given in the fourth
                              argument (the remains of the line).  It may return ‘501’ if the
                              third argument specifies a free-form header field name that is
                              invalid, or if body content extraction fails to succeed, ‘505’ if
                              any extracted address does not pass syntax and/or security checks
                              or on S-nail namespace violations, and ‘506’ to indicate prevention
                              of excessing a single-instance header — note that ‘Subject:’ can be
                              appended to (a space separator will be added automatically first).

                              ‘210’ is returned upon success, followed by the name of the header
                              and the list position of the newly inserted instance.  The list
                              position is always 1 for single-instance header fields.  All free-
                              form header fields are managed in a single list.

           attachment This command allows listing, removal and addition of message attachments.
                    The second argument specifies the subcommand to apply, one of:

                    list      List all attachments via ‘212’, or report ‘501’ if no attachments
                              exist.  This command is the default command of attachment if no
                              second argument has been given.

                    remove    This will remove the attachment given as the third argument, and
                              report ‘210’ upon success or ‘501’ if no such attachment can be
                              found.  If there exists any path component in the given argument,
                              then an exact match of the path which has been used to create the
                              attachment is used directly, but if only the basename of that path
                              matches then all attachments are traversed to find an exact match
                              first, and the removal occurs afterwards; if multiple basenames
                              match, a ‘506’ error occurs.  Message attachments are treated as
                              absolute pathnames.

                              If no path component exists in the given argument, then all
                              attachments will be searched for ‘filename=’ parameter matches as
                              well as for matches of the basename of the path which has been used
                              when the attachment has been created; multiple matches result in a
                              ‘506’.

                    remove-at This will interpret the third argument as a number and remove the
                              attachment at that list position (counting from one!), reporting
                              ‘210’ upon success or ‘505’ if the argument is not a number or
                              ‘501’ if no such attachment exists.

                    insert    Adds the attachment given as the third argument, specified exactly
                              as documented for the command line option -a, and supporting the
                              message number extension as documented for ~@.  This reports ‘210’
                              upon success, with the index of the new attachment following, ‘505’
                              if the given file cannot be opened, ‘506’ if an on-the-fly
                              performed character set conversion fails, otherwise ‘501’ is
                              reported; this is also reported if character set conversion is
                              requested but not available.

                    attribute This uses the same search mechanism as described for remove and
                              prints any known attributes of the first found attachment via ‘212’
                              upon success or ‘501’ if no such attachment can be found.  The
                              attributes are written as lines of keyword and value tuples, the
                              keyword being separated from the rest of the line with an ASCII SP
                              space character.

                    attribute-at This uses the same search mechanism as described for remove-at
                              and is otherwise identical to attribute.

                    attribute-set This uses the same search mechanism as described for remove,
                              and will assign the attribute given as the fourth argument, which
                              is expected to be a value tuple of keyword and other data,
                              separated by a ASCII SP space or TAB tabulator character.  If the
                              value part is empty, then the given attribute is removed, or reset
                              to a default value if existence of the attribute is crucial.

                              It returns via ‘210’ upon success, with the index of the found
                              attachment following, ‘505’ for message attachments or if the given
                              keyword is invalid, and ‘501’ if no such attachment can be found.
                              The following keywords may be used (case-insensitively):

                              ‘filename’  Sets the filename of the MIME part, i.e., the name that
                                          is used for display and when (suggesting a name for)
                                          saving (purposes).
                              ‘content-description’ Associate some descriptive information to the
                                          attachment's content, used in favour of the plain
                                          filename by some MUAs.
                              ‘content-id’ May be used for uniquely identifying MIME entities in
                                          several contexts; this expects a special reference
                                          address format as defined in RFC 2045 and generates a
                                          ‘505’ upon address content verification failure.
                              ‘content-type’ Defines the media type/subtype of the part, which is
                                          managed automatically, but can be overwritten.
                              ‘content-disposition’ Automatically set to the string ‘attachment’.

                    attribute-set-at This uses the same search mechanism as described for
                              remove-at and is otherwise identical to attribute-set.

     ~A    The same as ‘~i Sign’.

     ~a    The same as ‘~i sign’.

     ~b name ...
           Add the given names to the list of blind carbon copy recipients.

     ~c name ...
           Add the given names to the list of carbon copy recipients.

     ~d    Read the file specified by the DEAD variable into the message.

     ~e    Invoke the text EDITOR on the message collected so far, then return to compose mode.
           ~v can be used for a more display oriented editor, and ~|| offers a pipe-based editing
           approach.

     ~F messages
           Read the named messages into the message being sent, including all message headers and
           MIME parts.  If no messages are specified, read in the current message, the “dot”.

     ~f messages
           Read the named messages into the message being sent.  If no messages are specified,
           read in the current message, the “dot”.  Strips down the list of header fields
           according to the ‘type’ white- and blacklist selection of headerpick.  For MIME
           multipart messages, only the first displayable part is included.

     ~H    Edit the message header fields ‘From:’, ‘Reply-To:’ and ‘Sender:’ by typing each one
           in turn and allowing the user to edit the field.  The default values for these fields
           originate from the from, reply-to and sender variables.

     ~h    Edit the message header fields ‘To:’, ‘Cc:’, ‘Bcc:’ and ‘Subject:’ by typing each one
           in turn and allowing the user to edit the field.

     ~I variable
           Insert the value of the specified variable into the message.  The message remains
           unaltered if the variable is unset or empty.  Any embedded character sequences ‘\t’
           horizontal tabulator and ‘\n’ line feed are expanded in posix mode; otherwise the
           expansion should occur at set time ([v15 behaviour may differ] by using the command
           modifier wysh).

     ~i variable
           Like ~I, but appends a newline character.

     ~M messages
           Read the named messages into the message being sent, indented by indentprefix.  If no
           messages are specified, read the current message, the “dot”.

     ~m messages
           Read the named messages into the message being sent, indented by indentprefix.  If no
           messages are specified, read the current message, the “dot”.  Strips down the list of
           header fields according to the ‘type’ white- and blacklist selection of headerpick.
           For MIME multipart messages, only the first displayable part is included.

     ~p    Display the message collected so far, prefaced by the message header fields and
           followed by the attachment list, if any.

     ~Q    Read in the given / current message(s) according to the algorithm of quote.

     ~q    Abort the message being sent, copying it to the file specified by the DEAD variable if
           save is set.

     ~R filename
           Identical to ~r, but indent each line that has been read by indentprefix.

     ~r filename [HERE-delimiter]
           Read the named file, object to the usual Filename transformations, into the message;
           if (the expanded) filename is the hyphen-minus ‘-’ then standard input is used, e.g.,
           for pasting purposes.  Only in this latter mode HERE-delimiter may be given: if it is
           data will be read in until the given HERE-delimiter is seen on a line by itself, and
           encountering EOF is an error; the HERE-delimiter is a required argument in non-
           interactive mode; if it is single-quote quoted then the pasted content will not be
           expanded, [v15 behaviour may differ] otherwise a future version of S-nail may perform
           shell-style expansion on the content.

     ~s string
           Cause the named string to become the current subject field.  Newline (NL) and
           carriage-return (CR) bytes are invalid and will be normalized to space (SP)
           characters.

     ~t name ...
           Add the given name(s) to the direct recipient list.

     ~U messages
           Read in the given / current message(s) excluding all headers, indented by
           indentprefix.

     ~u messages
           Read in the given / current message(s), excluding all headers.

     ~v    Invoke the VISUAL editor on the message collected so far, then return to compose mode.
           ~e can be used for a less display oriented editor, and ~|| offers a pipe-based editing
           approach.

     ~w filename
           Write the message onto the named file, which is object to the usual Filename
           transformations.  If the file exists, the message is appended to it.

     ~x    Same as ~q, except that the message is not saved at all.

INTERNAL VARIABLES

     Internal S-nail variables are controlled via the set and unset commands; prefixing a
     variable name with the string ‘no’ and calling set has the same effect as using unset:
     ‘unset crt’ and ‘set nocrt’ do the same thing.  varshow will give more insight on the given
     variable(s), and set, when called without arguments, will show a listing of all variables.
     Both commands support a more verbose listing mode.  Some well-known variables will also
     become inherited from the program ENVIRONMENT implicitly, others can be imported explicitly
     with the command environ and henceforth share said properties.

     Two different kinds of internal variables exist, and both of which can also form chains.
     There are boolean variables, which can only be in one of the two states “set” and “unset”,
     and value variables with a(n optional) string value.  For the latter proper quoting is
     necessary upon assignment time, the introduction of the section COMMANDS documents the
     supported quoting rules.

           ? wysh set one=val\ 1 two="val 2" \
               three='val "3"' four=$'val \'4\''; \
               varshow one two three four; \
               unset one two three four

     Dependent upon the actual option string values may become interpreted as colour names,
     command specifications, normal text, etc.  They may be treated as numbers, in which case
     decimal values are expected if so documented, but otherwise any numeric format and base that
     is valid and understood by the vexpr command may be used, too.

     There also exists a special kind of string value, the “boolean string”, which must either be
     a decimal integer (in which case ‘0’ is false and ‘1’ and any other value is true) or any of
     the (case-insensitive) strings ‘off’, ‘no’, ‘n’ and ‘false’ for a false boolean and ‘on’,
     ‘yes’, ‘y’ and ‘true’ for a true boolean; a special kind of boolean string is the
     “quadoption”, which is a boolean string that can optionally be prefixed with the (case-
     insensitive) term ‘ask-’, as in ‘ask-yes’, which causes prompting of the user in interactive
     mode, with the given boolean as the default value.

     Variable chains extend a plain ‘variable’ with ‘variable-HOST’ and ‘variable-USER@HOST’
     variants.  Here ‘HOST’ will be converted to all lowercase when looked up (but not when the
     variable is set or unset!), [Option]ally IDNA converted, and indeed means ‘server:port’ if a
     ‘port’ had been specified in the contextual Uniform Resource Locator URL, see On URL syntax
     and credential lookup.  Even though this mechanism is based on URLs no URL percent encoding
     may be applied to neither of ‘USER’ nor ‘HOST’, variable chains need to be specified using
     raw data; the mentioned section contains examples.  Variables which support chains are
     explicitly documented as such, and S-nail treats the base name of any such variable special,
     meaning that users should not create custom names like ‘variable-xyz’ in order to avoid
     false classifications and treatment of such variables.

   Initial settings
     The standard POSIX 2008/Cor 2-2016 mandates the following initial variable settings:
     noallnet, noappend, asksub, noaskbcc, noautoprint, nobang, nocmd, nocrt, nodebug, nodot,
     escape set to ‘~’, noflipr, nofolder, header, nohold, noignore, noignoreeof, nokeep,
     nokeepsave, nometoo, nooutfolder, nopage, prompt set to ‘? ’, noquiet, norecord, save,
     nosendwait, noshowto, noSign, nosign, toplines set to ‘5’.

     Notes: S-nail does not support the noonehop variable – use command line options or
     mta-arguments to pass options through to a mta.  And the default global /etc/s-nail.rc file,
     which is loaded unless the -: (with according argument) or -n command line options have been
     used, or the MAILX_NO_SYSTEM_RC environment variable is set (see Resource files) bends those
     initial settings a bit, e.g., it sets the variables hold, keepsave and keep, to name a few,
     establishes a default headerpick selection etc., and should thus be taken into account.

   Variables
     ?     (Read-only) The exit status of the last command, or the return value of the macro
           called last.  This status has a meaning in the state machine: in conjunction with
           errexit any non-0 exit status will cause a program exit, and in posix mode any error
           while loading (any of the) resource files will have the same effect.  ignerr, one of
           the Command modifiers, can be used to instruct the state machine to ignore errors.

     !     (Read-only) The current error number (errno(3)), which is set after an error occurred;
           it is also available via ^ERR, and the error name and documentation string can be
           queried via ^ERRNAME and ^ERRDOC.  [v15 behaviour may differ] This machinery is new
           and the error number is only really usable if a command explicitly states that it
           manages the variable !, for others errno will be used in case of errors, or ^ERR-INVAL
           if that is 0: it thus may or may not reflect the real error.  The error number may be
           set with the command return.

     ^     (Read-only) This is a multiplexer variable which performs dynamic expansion of the
           requested state or condition, of which there are:

           ^ERR, ^ERRDOC, ^ERRNAME
                 The number, documentation, and name of the current errno(3), respectively, which
                 is usually set after an error occurred.  The documentation is an [Option], the
                 name is used if not available.  [v15 behaviour may differ] This machinery is new
                 and is usually reliable only if a command explicitly states that it manages the
                 variable !, which is effectively identical to ^ERR.  Each of those variables can
                 be suffixed with a hyphen minus followed by a name or number, in which case the
                 expansion refers to the given error.  Note this is a direct mapping of (a subset
                 of) the system error values:

                       define work {
                         eval echo \$1: \$^ERR-$1:\
                           \$^ERRNAME-$1: \$^ERRDOC-$1
                         vput vexpr i + "$1" 1
                         if [ $i -lt 16 ]
                           \xcall work $i
                         end
                       }
                       call work 0

     *     (Read-only) Expands all positional parameters (see 1), separated by the first
           character of the value of ifs.  [v15 behaviour may differ] The special semantics of
           the equally named special parameter of the sh(1) are not yet supported.

     @     (Read-only) Expands all positional parameters (see 1), separated by a space character.
           If placed in double quotation marks, each positional parameter is properly quoted to
           expand to a single parameter again.

     #     (Read-only) Expands to the number of positional parameters, i.e., the size of the
           positional parameter stack in decimal.

     0     (Read-only) Inside the scope of a defined and called macro this expands to the name of
           the calling macro, or to the empty string if the macro is running from top-level.  For
           the [Option]al regular expression search and replace operator of vexpr this expands to
           the entire matching expression.  It represents the program name in global context.

     1     (Read-only) Access of the positional parameter stack.  All further parameters can be
           accessed with this syntax, too, e.g., ‘2’, ‘3’ etc.; positional parameters can be
           shifted off the stack by calling shift.  The parameter stack contains, e.g., the
           arguments of a called defined macro, the matching groups of the [Option]al regular
           expression search and replace expression of vexpr, and can be explicitly created or
           overwritten with the command vpospar.

     account
           (Read-only) Is set to the active account.

     add-file-recipients
           (Boolean) When file or pipe recipients have been specified, mention them in the
           corresponding address fields of the message instead of silently stripping them from
           their recipient list.  By default such addressees are not mentioned.

     allnet
           (Boolean) Causes only the local part to be evaluated when comparing addresses.

     append
           (Boolean) Causes messages saved in the secondary mailbox MBOX to be appended to the
           end rather than prepended.  This should always be set.

     askatend
           (Boolean) Causes the prompts for ‘Cc:’ and ‘Bcc:’ lists to appear after the message
           has been edited.

     askattach
           (Boolean) If set, S-nail asks for files to attach at the end of each message.  An
           empty line finalizes the list.

     askcc
           (Boolean) Causes the user to be prompted for carbon copy recipients (at the end of
           each message if askatend or bsdcompat are set).

     askbcc
           (Boolean) Causes the user to be prompted for blind carbon copy recipients (at the end
           of each message if askatend or bsdcompat are set).

     asksend
           (Boolean) Causes the user to be prompted for confirmation to send the message or
           reenter compose mode after having been shown an envelope summary.  This is by default
           enabled.

     asksign
           (Boolean)[Option] Causes the user to be prompted if the message is to be signed at the
           end of each message.  The smime-sign variable is ignored when this variable is set.

     asksub
           (Boolean) Causes S-nail to prompt for the subject upon entering compose mode unless a
           subject already exists.

     attrlist
           A sequence of characters to display in the ‘attribute’ column of the headline as shown
           in the display of headers; each for one type of messages (see Message states), with
           the default being ‘NUROSPMFAT+-$~’ or ‘NU  *HMFAT+-$~’ if the bsdflags variable is
           set, in the following order:

           ‘N’  new.
           ‘U’  unread but old.
           ‘R’  new but read.
           ‘O’  read and old.
           ‘S’  saved.
           ‘P’  preserved.
           ‘M’  mboxed.
           ‘F’  flagged.
           ‘A’  answered.
           ‘T’  draft.
           ‘+’  [v15 behaviour may differ] start of a (collapsed) thread in threaded mode (see
                autosort, thread);
           ‘-’  [v15 behaviour may differ] an uncollapsed thread in threaded mode; only used in
                conjunction with -L.
           ‘$’  classified as spam.
           ‘~’  classified as possible spam.

     autobcc
           Specifies a list of recipients to which a blind carbon copy of each outgoing message
           will be sent automatically.

     autocc
           Specifies a list of recipients to which a carbon copy of each outgoing message will be
           sent automatically.

     autocollapse
           (Boolean) Causes threads to be collapsed automatically when .Ql thread Ns ed sort mode
           is entered (see the collapse command).

     autoprint
           (Boolean) Enable automatic typeing of a(n existing) “successive” message after delete
           and undelete commands, e.g., the message that becomes the new “dot” is shown
           automatically, as via dp or dt.

     autosort
           Causes sorted mode (see the sort command) to be entered automatically with the value
           of this variable as sorting method when a folder is opened, e.g., ‘set
           autosort=thread’.

     bang  (Boolean) Enables the substitution of all not (reverse-solidus) escaped exclamation
           mark ‘!’ characters by the contents of the last executed command for the ! shell
           escape command and ~!, one of the compose mode COMMAND ESCAPES.  If this variable is
           not set no reverse solidus stripping is performed.

     bind-timeout
           [Option] Terminals generate multi-byte sequences for certain forms of input, for
           example for function and other special keys.  Some terminals however do not write
           these multi-byte sequences as a whole, but byte-by-byte, and the latter is what S-nail
           actually reads.  This variable specifies the timeout in milliseconds that the MLE (see
           On terminal control and line editor) waits for more bytes to arrive unless it
           considers a sequence “complete”.  The default is 200.

     bsdcompat
           (Boolean) Sets some cosmetical features to traditional BSD style; has the same affect
           as setting askatend and all other variables prefixed with ‘bsd’; it also changes the
           behaviour of emptystart (which does not exist in BSD).

     bsdflags
           (Boolean) Changes the letters shown in the first column of a header summary to
           traditional BSD style.

     bsdheadline
           (Boolean) Changes the display of columns in a header summary to traditional BSD style.

     bsdmsgs
           (Boolean) Changes some informational messages to traditional BSD style.

     bsdorder
           (Boolean) Causes the ‘Subject:’ field to appear immediately after the ‘To:’ field in
           message headers and with the ~h COMMAND ESCAPES.

     build-cc, build-ld, build-os, build-rest
           (Read-only) The build environment, including the compiler, the linker, the operating
           system S-nail has been build for, usually taken from uname(1) via ‘uname -s’, and then
           lowercased, as well as all the rest that may possibly be useful to include in a bug
           report, respectively.

     charset-7bit
           The value that should appear in the ‘charset=’ parameter of ‘Content-Type:’ MIME
           header fields when no character set conversion of the message data was performed.
           This defaults to US-ASCII, and the chosen character set should be US-ASCII compatible.

     charset-8bit
           [Option] The default 8-bit character set that is used as an implicit last member of
           the variable sendcharsets.  This defaults to UTF-8 if character set conversion
           capabilities are available, and to ISO-8859-1 otherwise (unless the operating system
           environment is known to always and exclusively support UTF-8 locales), in which case
           the only supported character set is ttycharset and this variable is effectively
           ignored.  Refer to the section Character sets for the complete picture of character
           set conversion in S-nail.

     charset-unknown-8bit
           [Option] RFC 1428 specifies conditions when internet mail gateways shall “upgrade” the
           content of a mail message by using a character set with the name ‘unknown-8bit’.
           Because of the unclassified nature of this character set S-nail will not be capable to
           convert this character set to any other character set.  If this variable is set any
           message part which uses the character set ‘unknown-8bit’ is assumed to really be in
           the character set given in the value, otherwise the (final) value of charset-8bit is
           used for this purpose.

           This variable will also be taken into account if a MIME type (see The mime.types
           files) of a MIME message part that uses the ‘binary’ character set is forcefully
           treated as text.

     cmd   The default value for the pipe command.

     colour-disable
           (Boolean)[Option] Forcefully disable usage of colours.  Also see the section Coloured
           display.

     colour-pager
           (Boolean)[Option] Whether colour shall be used for output that is paged through PAGER.
           Note that pagers may need special command line options, e.g., less(1) requires the
           option -R and lv(1) the option -c in order to support colours.  Often doing manual
           adjustments is unnecessary since S-nail may perform adjustments dependent on the value
           of the environment variable PAGER (see there for more).

     contact-mail, contact-web
           (Read-only) Addresses for contact per email and web, respectively, e.g., for bug
           reports, suggestions, or help regarding S-nail.  The former can be used directly: ‘?
           eval mail $contact-mail’.

     crt   In a(n interactive) terminal session, then if this valued variable is set it will be
           used as a threshold to determine how many lines the given output has to span before it
           will be displayed via the configured PAGER; Usage of the PAGER can be forced by
           setting this to the value ‘0’, setting it without a value will deduce the current
           height of the terminal screen to compute the threshold (see LINES, screen and
           stty(1)).  [v15 behaviour may differ] At the moment this uses the count of lines of
           the message in wire format, which, dependent on the mime-encoding of the message, is
           unrelated to the number of display lines.  (The software is old and historically the
           relation was a given thing.)

     customhdr
           Define a set of custom headers to be injected into newly composed or forwarded
           messages.  A custom header consists of the field name followed by a colon ‘:’ and the
           field content body.  Standard header field names cannot be overwritten by a custom
           header.  Different to the command line option -C the variable value is interpreted as
           a comma-separated list of custom headers: to include commas in header bodies they need
           to become escaped with reverse solidus ‘\’.  Headers can be managed more freely in
           compose mode via ~^.

                 ? set customhdr='Hdr1: Body1-1\, Body1-2, Hdr2: Body2'

     datefield
           Controls the appearance of the ‘%d’ date and time format specification of the headline
           variable, that is used, for example, when viewing the summary of headers.  If unset,
           then the local receiving date is used and displayed unformatted, otherwise the message
           sending ‘Date:’.  It is possible to assign a strftime(3) format string and control
           formatting, but embedding newlines via the ‘%n’ format is not supported, and will
           result in display errors.  The default is ‘%Y-%m-%d %H:%M’, and also see
           datefield-markout-older.

     datefield-markout-older
           Only used in conjunction with datefield.  Can be used to create a visible distinction
           of messages dated more than a day in the future, or older than six months, a concept
           comparable to the -l option of the POSIX utility ls(1).  If set to the empty string,
           then the plain month, day and year of the ‘Date:’ will be displayed, but a strftime(3)
           format string to control formatting can be assigned.  The default is ‘%Y-%m-%d’.

     debug
           (Boolean) Enables debug messages and obsoletion warnings, disables the actual delivery
           of messages and also implies norecord as well as nosave.

     disposition-notification-send
           (Boolean)[Option] Emit a ‘Disposition-Notification-To:’ header (RFC 3798) with the
           message.  This requires the from variable to be set.

     dot   (Boolean) When dot is set, a period ‘.’ on a line by itself during message input in
           (interactive or batch -#) compose mode will be treated as end-of-message (in addition
           to the normal end-of-file condition).  This behaviour is implied in posix mode with a
           set ignoreeof.

     dotlock-disable
           (Boolean)[Option] Disable creation of dotlock files for MBOX databases.

     dotlock-ignore-error
           [Obsolete](Boolean)[Option] Ignore failures when creating dotlock files.  Please use
           dotlock-disable instead.

     editalong
           If this variable is set then the editor is started automatically when a message is
           composed in interactive mode.  If the value starts with the letter ‘v’ then this acts
           as if ~v, otherwise as if ~e (see COMMAND ESCAPES) had been specified.  The
           editheaders variable is implied for this automatically spawned editor session.

     editheaders
           (Boolean) When a message is edited while being composed, its header is included in the
           editable text.

     emptystart
           (Boolean) When entering interactive mode S-nail normally writes “No mail for user” and
           exits immediately if a mailbox is empty or does not exist.  If this variable is set
           S-nail starts even with an empty or non-existent mailbox (the latter behaviour
           furtherly depends upon bsdcompat, though).

     errexit
           (Boolean) Let each command with a non-0 exit status, including every called macro
           which returns a non-0 status, cause a program exit unless prefixed by ignerr (see
           Command modifiers).  This also affects COMMAND ESCAPES, but which use a different
           modifier for ignoring the error.  Please refer to the variable ? for more on this
           topic.

     escape
           The first character of this value defines the escape character for COMMAND ESCAPES in
           compose mode.  The default value is the character tilde ‘~’.  If set to the empty
           string, command escapes are disabled.

     expandaddr
           If unset then file and command pipeline address targets are not allowed, and any such
           address will be filtered out, giving a warning message.  If set then all possible
           recipient address specifications will be accepted unless a possible value content is
           more specific (also see On sending mail, and non-interactive mode); if desired so only
           in interactive mode, or when tilde commands were enabled explicitly via -~ or -#, the
           (case-insensitive) value ‘restrict’ can be used (this really acts like
           ‘restrict,-all,+name,+addr’, so that care for ordering issues must be taken).

           The value is actually interpreted as a comma-separated list.  If that contains ‘fail’
           the existence of disallowed addressees is treated as a hard send error instead of only
           causing them to be filtered out.  Address targets can be added and subtracted by
           prefixing with a plus sign ‘+’ or hyphen-minus ‘-’ prefix: the value ‘all’ addresses
           all possible address specifications, ‘file’ file targets, ‘pipe’ command pipeline
           targets, ‘name’ plain user names and (MTA) aliases and ‘addr’ network addresses;
           Targets are interpreted in the given order, so that ‘restrict,fail,+file,-all,+addr’
           will cause hard errors for any non-network address recipient address unless S-nail is
           in interactive mode or has been started with the -~ or -# command line option; in the
           latter case(s) any address may be used, then.

           Historically invalid network addressees were silently stripped off.  To change this so
           that any encountered invalid email address causes a hard error it must be ensured that
           ‘failinvaddr’ is an entry in the above list, which automatically enables network
           addressees; it really acts like ‘failinvaddr,+addr’, so that care for ordering issues
           must be taken.  [v15 behaviour may differ] If the value ‘shquote’ is present a few
           address providers (for example -b, -c and all recipients given on the command line)
           will be will evaluated as if specified within dollar-single-quotes (see Shell-style
           argument quoting).

     expandargv
           Unless this variable is set additional mta (Mail-Transfer-Agent) arguments from the
           command line, as can be given after a -- separator, results in a program termination
           with failure status.  The same can be accomplished by using the special (case-
           insensitive) value ‘fail’.  A lesser strict variant is the otherwise identical
           ‘restrict’, which does accept such arguments in interactive mode, or if tilde commands
           were enabled explicitly by using one of the command line options -~ or -#.  The empty
           value will allow unconditional usage.

     features
           (Read-only) String giving a list of optional features.  Features are preceded with a
           plus sign ‘+’ if they are available, with a hyphen-minus ‘-’ otherwise.  The output of
           the command version will include this information in a more pleasant output.

     flipr
           (Boolean) This setting reverses the meanings of a set of reply commands, turning the
           lowercase variants, which by default address all recipients included in the header of
           a message (reply, respond, followup) into the uppercase variants, which by default
           address the sender only (Reply, Respond, Followup) and vice versa.  The commands
           replysender, respondsender, followupsender as well as replyall, respondall,
           followupall are not affected by the current setting of flipr.

     folder
           The default path under which mailboxes are to be saved: filenames that begin with the
           plus sign ‘+’ will have the plus sign replaced with the value of this variable if set,
           otherwise the plus sign will remain unchanged when doing Filename transformations;
           also see file for more on this topic, and know about standard imposed implications of
           outfolder.  The value supports a subset of transformations itself, and if the non-
           empty value does not start with a solidus ‘/’, then the value of HOME will be prefixed
           automatically.  Once the actual value is evaluated first, the internal variable
           folder-resolved will be updated for caching purposes.

     folder-hook-FOLDER, folder-hook
           Names a defined macro which will be called whenever a file is opened.  The macro will
           also be invoked when new mail arrives, but message lists for commands executed from
           the macro only include newly arrived messages then.  localopts are activated by
           default in a folder hook, causing the covered settings to be reverted once the folder
           is left again.

           The specialized form will override the generic one if ‘FOLDER’ matches the file that
           is opened.  Unlike other folder specifications, the fully expanded name of a folder,
           without metacharacters, is used to avoid ambiguities.  However, if the mailbox resides
           under folder then the usual ‘+’ specification is tried in addition, e.g., if folder is
           “mail” (and thus relative to the user's home directory) then /home/usr1/mail/sent will
           be tried as ‘folder-hook-/home/usr1/mail/sent’ first, but then followed by
           ‘folder-hook-+sent’.

     folder-resolved
           (Read-only) Set to the fully resolved path of folder once that evaluation has
           occurred; rather internal.

     followup-to
           (Boolean) Controls whether a ‘Mail-Followup-To:’ header is generated when sending
           messages to known mailing lists.  Also see followup-to-honour and the commands mlist,
           mlsubscribe, reply and Lreply.

     followup-to-honour
           Controls whether a ‘Mail-Followup-To:’ header is honoured when group-replying to a
           message via reply or Lreply.  This is a quadoption; if set without a value it defaults
           to “yes”.  Also see followup-to and the commands mlist and mlsubscribe.

     forward-as-attachment
           (Boolean) Original messages are normally sent as inline text with the forward command,
           and only the first part of a multipart message is included.  With this setting enabled
           messages are sent as unmodified MIME ‘message/rfc822’ attachments with all of their
           parts included.

     forward-inject-head, forward-inject-tail
           The strings to put before and after the text of a message with the forward command,
           respectively.  The former defaults to ‘-------- Original Message --------\n’.  Special
           format directives in these strings will be expanded if possible, and if so configured
           the output will be folded according to quote-fold; for more please refer to
           quote-inject-head.  These variables are ignored if the forward-as-attachment variable
           is set.

     from  The address (or a list of addresses) to put into the ‘From:’ field of the message
           header, quoting RFC 5322: the author(s) of the message, that is, the mailbox(es) of
           the person(s) or system(s) responsible for the writing of the message.  According to
           that RFC setting the sender variable is required if from contains more than one
           address.  Dependent on the context these addresses are handled as if they were in the
           list of alternates.

           If a file-based MTA is used, then from (or, if that contains multiple addresses,
           sender) can nonetheless be enforced to appear as the envelope sender address at the
           MTA protocol level (the RFC 5321 reverse-path), either by using the -r command line
           option (with an empty argument; see there for the complete picture on this topic), or
           by setting the internal variable r-option-implicit.

           If the machine's hostname is not valid at the Internet (for example at a dialup
           machine) then either this variable or hostname ([v15-compat] a SMTP-based mta adds
           even more fine-tuning capabilities with smtp-hostname) have to be set: if so the
           message and MIME part related unique ID fields ‘Message-ID:’ and ‘Content-ID:’ will be
           created (except when disallowed by message-id-disable or stealthmua).

     fullnames
           (Boolean) Due to historical reasons comments and name parts of email addresses are
           removed by default when sending mail, replying to or forwarding a message.  If this
           variable is set such stripping is not performed.

     fwdheading
           [Obsolete] Predecessor of forward-inject-head.

     header
           (Boolean) Causes the header summary to be written at startup and after commands that
           affect the number of messages or the order of messages in the current folder.  Unless
           in posix mode a header summary will also be displayed on folder changes.  The command
           line option -N can be used to set noheader.

     headline
           A format string to use for the summary of headers.  Format specifiers in the given
           string start with a percent sign ‘%’ and may be followed by an optional decimal number
           indicating the field width — if that is negative, the field is to be left-aligned.
           Names and addresses are subject to modifications according to showname and showto.
           Valid format specifiers are:

           ‘%%’    A plain percent sign.
           ‘%>’    “Dotmark”: a space character but for the current message (“dot”), for which it
                   expands to ‘>’ (dependent on headline-plain).
           ‘%<’    “Dotmark”: a space character but for the current message (“dot”), for which it
                   expands to ‘<’ (dependent on headline-plain).
           ‘%$’    [Option] The spam score of the message, as has been classified via the command
                   spamrate.  Shows only a replacement character if there is no spam support.
           ‘%a’    Message attribute character (status flag); the actual content can be adjusted
                   by setting attrlist.
           ‘%d’    The date found in the ‘Date:’ header of the message when datefield is set (the
                   default), otherwise the date when the message was received.  Formatting can be
                   controlled by assigning a strftime(3) format string to datefield (and
                   datefield-markout-older).
           ‘%e’    The indenting level in ‘thread’ed sort mode.
           ‘%f’    The address of the message sender.
           ‘%i’    The message thread tree structure.  (Note that this format does not support a
                   field width, and honours headline-plain.)
           ‘%l’    The number of lines of the message, if available.
           ‘%m’    Message number.
           ‘%o’    The number of octets (bytes) in the message, if available.
           ‘%s’    Message subject (if any).
           ‘%S’    Message subject (if any) in double quotes.
           ‘%T’    Message recipient flags: is the addressee of the message a known or subscribed
                   mailing list – see mlist and mlsubscribe.
           ‘%t’    The position in threaded/sorted order.
           ‘U’     The value 0 except in an IMAP mailbox, where it expands to the UID of the
                   message.

           The default is ‘%>%a%m %-18f %16d %4l/%-5o %i%-s’, or
           ‘%>%a%m %20-f  %16d %3l/%-5o %i%-S’ if bsdcompat is set.  Also see attrlist,
           headline-plain and headline-bidi.

     headline-bidi
           Bidirectional text requires special treatment when displaying headers, because numbers
           (in dates or for file sizes etc.) will not affect the current text direction, in
           effect resulting in ugly line layouts when arabic or other right-to-left text is to be
           displayed.  On the other hand only a minority of terminals is capable to correctly
           handle direction changes, so that user interaction is necessary for acceptable
           results.  Note that extended host system support is required nonetheless, e.g.,
           detection of the terminal character set is one precondition; and this feature only
           works in an Unicode (i.e., UTF-8) locale.

           In general setting this variable will cause S-nail to encapsulate text fields that may
           occur when displaying headline (and some other fields, like dynamic expansions in
           prompt) with special Unicode control sequences; it is possible to fine-tune the
           terminal support level by assigning a value: no value (or any value other than ‘1’,
           ‘2’ and ‘3’) will make S-nail assume that the terminal is capable to properly deal
           with Unicode version 6.3, in which case text is embedded in a pair of U+2068 (FIRST
           STRONG ISOLATE) and U+2069 (POP DIRECTIONAL ISOLATE) characters.  In addition no space
           on the line is reserved for these characters.

           Weaker support is chosen by using the value ‘1’ (Unicode 6.3, but reserve the room of
           two spaces for writing the control sequences onto the line).  The values ‘2’ and ‘3’
           select Unicode 1.1 support (U+200E, LEFT-TO-RIGHT MARK); the latter again reserves
           room for two spaces in addition.

     headline-plain
           (Boolean) On Unicode (UTF-8) aware terminals enhanced graphical symbols are used by
           default for certain entries of headline.  If this variable is set only basic US-ASCII
           symbols will be used.

     history-file
           [Option] If a line editor is available then this can be set to name the (expandable)
           path of the location of a permanent history file; also see history-size.

     history-gabby
           (Boolean)[Option] Add more entries to the history as is normally done.

     history-gabby-persist
           (Boolean)[Option] S-nail's own MLE will not save the additional history-gabby entries
           in persistent storage unless this variable is set.  On the other hand it will not
           loose the knowledge of whether a persistent entry was gabby or not.  Also see
           history-file.

     history-size
           [Option] Setting this variable imposes a limit on the number of concurrent history
           entries.  If set to the value 0 then no further history entries will be added, and
           loading and incorporation of the history-file upon program startup can also be
           suppressed by doing this.  Runtime changes will not be reflected before the history is
           saved or loaded (again).

     hold  (Boolean) This setting controls whether messages are held in the system inbox, and it
           is set by default.

     hostname
           Used instead of the value obtained from uname(3) and getaddrinfo(3) as the hostname
           when expanding local addresses, e.g., in ‘From:’ (also see On sending mail, and non-
           interactive mode, especially for expansion of network addresses that contain domain-
           less valid user names in angle brackets).  If either of from or this variable Is set
           the message and MIME part related unique ID fields ‘Message-ID:’ and ‘Content-ID:’
           will be created (except when disallowed by message-id-disable or stealthmua).  If the
           [Option]al IDNA support is available (see idna-disable) variable assignment is aborted
           when a necessary conversion fails.

           Setting it to the empty string will cause the normal hostname to be used, but
           nonetheless enables creation of said ID fields.  [v15-compat] in conjunction with the
           built-in SMTP mta smtp-hostname also influences the results: one should produce some
           test messages with the desired combination of hostname, and/or from, sender etc.
           first.

     idna-disable
           (Boolean)[Option] Can be used to turn off the automatic conversion of domain names
           according to the rules of IDNA (internationalized domain names for applications).
           Since the IDNA code assumes that domain names are specified with the ttycharset
           character set, an UTF-8 locale charset is required to represent all possible
           international domain names (before conversion, that is).

     ifs   The input field separator that is used ([v15 behaviour may differ] by some functions)
           to determine where to split input data.

           1.   Unsetting is treated as assigning the default value, ‘ \t\n’.
           2.   If set to the empty value, no field splitting will be performed.
           3.   If set to a non-empty value, all whitespace characters are extracted and assigned
                to the variable ifs-ws.

           a.   ifs-ws will be ignored at the beginning and end of input.  Diverging from POSIX
                shells default whitespace is removed in addition, which is owed to the entirely
                different line content extraction rules.
           b.   Each occurrence of a character of ifs will cause field-splitting, any adjacent
                ifs-ws characters will be skipped.

     ifs-ws
           (Read-only) Automatically deduced from the whitespace characters in ifs.

     ignore
           (Boolean) Ignore interrupt signals from the terminal while entering messages; instead
           echo them as ‘@’ characters and discard the current line.

     ignoreeof
           (Boolean) Ignore end-of-file conditions (‘control-D’) in compose mode on message input
           and in interactive command input.  If set an interactive command input session can
           only be left by explicitly using one of the commands exit and quit, and message input
           in compose mode can only be terminated by entering a period ‘.’ on a line by itself or
           by using the ~. COMMAND ESCAPES; Setting this implies the behaviour that dot describes
           in posix mode.

     inbox
           If this is set to a non-empty string it will specify the user's primary system
           mailbox, overriding MAIL and the system-dependent default, and (thus) be used to
           replace ‘%’ when doing Filename transformations; also see file for more on this topic.
           The value supports a subset of transformations itself.

     indentprefix
           String used by the ~m, ~M and ~R COMMAND ESCAPES and by the quote option for indenting
           messages, in place of the POSIX mandated default tabulator character ‘\t’.  Also see
           quote-chars.

     keep  (Boolean) If set, an empty primary system mailbox file is not removed.  Note that, in
           conjunction with posix mode any empty file will be removed unless this variable is
           set.  This may improve the interoperability with other mail user agents when using a
           common folder directory, and prevents malicious users from creating fake mailboxes in
           a world-writable spool directory.  [v15 behaviour may differ] Only local regular
           (MBOX) files are covered, Maildir and other mailbox types will never be removed, even
           if empty.

     keep-content-length
           (Boolean) When (editing messages and) writing MBOX mailbox files S-nail can be told to
           keep the ‘Content-Length:’ and ‘Lines:’ header fields that some MUAs generate by
           setting this variable.  Since S-nail does neither use nor update these non-
           standardized header fields (which in itself shows one of their conceptual problems),
           stripping them should increase interoperability in between MUAs that work with with
           same mailbox files.  Note that, if this is not set but writebackedited, as below, is,
           a possibly performed automatic stripping of these header fields already marks the
           message as being modified.  [v15 behaviour may differ] At some future time S-nail will
           be capable to rewrite and apply an mime-encoding to modified messages, and then those
           fields will be stripped silently.

     keepsave
           (Boolean) When a message is saved it is usually discarded from the originating folder
           when S-nail is quit.  This setting causes all saved message to be retained.

     line-editor-disable
           (Boolean) Turn off any enhanced line editing capabilities (see On terminal control and
           line editor for more).

     line-editor-no-defaults
           (Boolean)[Option] Do not establish any default key binding.

     log-prefix
           Error log message prefix string (‘s-nail: ’).

     mailbox-display
           (Read-only) The name of the current mailbox (file), possibly abbreviated for display
           purposes.

     mailbox-resolved
           (Read-only) The fully resolved path of the current mailbox.

     mailx-extra-rc
           An additional startup file that is loaded as the last of the Resource files.  Use this
           file for commands that are not understood by other POSIX mailx(1) implementations,
           i.e., mostly anything which is not covered by Initial settings.

     markanswered
           (Boolean) When a message is replied to and this variable is set, it is marked as
           having been answered.  See the section Message states.

     mbox-rfc4155
           (Boolean) When opening MBOX mailbox databases S-nail by default uses tolerant POSIX
           rules for detecting message boundaries (so-called ‘From_’ lines) due to compatibility
           reasons, instead of the stricter rules that have been standardized in RFC 4155.  This
           behaviour can be switched to the stricter RFC 4155 rules by setting this variable.
           (This is never necessary for any message newly generated by S-nail, it only applies to
           messages generated by buggy or malicious MUAs, or may occur in old MBOX databases:
           S-nail itself will choose a proper mime-encoding to avoid false interpretation of
           ‘From_’ content lines in the MBOX database.)

           This may temporarily be handy when S-nail complains about invalid ‘From_’ lines when
           opening a MBOX: in this case setting this variable and re-opening the mailbox in
           question may correct the result.  If so, copying the entire mailbox to some other
           file, as in ‘copy * SOME-FILE’, will perform proper, all-compatible ‘From_’ quoting
           for all detected messages, resulting in a valid MBOX mailbox.  Finally the variable
           can be unset again:

                 define mboxfix {
                   localopts yes; wysh set mbox-rfc4155;\
                     wysh File "${1}"; eval copy * "${2}"
                 }
                 call mboxfix /tmp/bad.mbox /tmp/good.mbox

     memdebug
           (Boolean) Internal development variable.

     message-id-disable
           (Boolean) By setting this variable the generation of ‘Message-ID:’ and ‘Content-ID:’
           message and MIME part headers can be completely suppressed, effectively leaving this
           task up to the mta (Mail-Transfer-Agent) or the SMTP server.  Note that according to
           RFC 5321 a SMTP server is not required to add this field by itself, so it should be
           ensured that it accepts messages without ‘Message-ID’.

     message-inject-head
           A string to put at the beginning of each new message, followed by a newline.
           [Obsolete] The escape sequences tabulator ‘\t’ and newline ‘\n’ are understood (use
           the wysh prefix when setting the variable(s) instead).

     message-inject-tail
           A string to put at the end of each new message, followed by a newline.  [Obsolete] The
           escape sequences tabulator ‘\t’ and newline ‘\n’ are understood (use the wysh prefix
           when setting the variable(s) instead).

     metoo
           (Boolean) Usually, when an alias expansion contains the sender, the sender is removed
           from the expansion.  Setting this option suppresses these removals.  Note that a set
           metoo also causes a ‘-m’ option to be passed through to the mta (Mail-Transfer-Agent);
           though most of the modern MTAs no longer document this flag, no MTA is known which
           does not support it (for historical compatibility).

     mime-allow-text-controls
           (Boolean) When sending messages, each part of the message is MIME-inspected in order
           to classify the ‘Content-Type:’ and ‘Content-Transfer-Encoding:’ (see mime-encoding)
           that is required to send this part over mail transport, i.e., a computation rather
           similar to what the file(1) command produces when used with the ‘--mime’ option.

           This classification however treats text files which are encoded in UTF-16 (seen for
           HTML files) and similar character sets as binary octet-streams, forcefully changing
           any ‘text/plain’ or ‘text/html’ specification to ‘application/octet-stream’: If that
           actually happens a yet unset charset MIME parameter is set to ‘binary’, effectively
           making it impossible for the receiving MUA to automatically interpret the contents of
           the part.

           If this variable is set, and the data was unambiguously identified as text data at
           first glance (by a ‘.txt’ or ‘.html’ file extension), then the original
           ‘Content-Type:’ will not be overwritten.

     mime-alternative-favour-rich
           (Boolean) If this variable is set then rich MIME alternative parts (e.g., HTML) will
           be preferred in favour of included plain text versions when displaying messages,
           provided that a handler exists which produces output that can be (re)integrated into
           S-nail's normal visual display.  (E.g., at the time of this writing some newsletters
           ship their full content only in the rich HTML part, whereas the plain text part only
           contains topic subjects.)

     mime-counter-evidence
           Normally the ‘Content-Type:’ field is used to decide how to handle MIME parts.  Some
           MUAs, however, do not use The mime.types files (also see HTML mail and MIME
           attachments) or a similar mechanism to correctly classify content, but specify an
           unspecific MIME type (‘application/octet-stream’) even for plain text attachments.  If
           this variable is set then S-nail will try to re-classify such MIME message parts, if
           possible, for example via a possibly existing attachment filename.  A non-empty value
           may also be given, in which case a number is expected, actually a carrier of bits,
           best specified as a binary value, e.g., ‘0b1111’.

           ·   If bit two is set (counting from 1, decimal 2) then the detected mimetype will be
               carried along with the message and be used for deciding which MIME handler is to
               be used, for example; when displaying such a MIME part the part-info will indicate
               the overridden content-type by showing a plus sign ‘+’.
           ·   If bit three is set (decimal 4) then the counter-evidence is always produced and a
               positive result will be used as the MIME type, even forcefully overriding the
               parts given MIME type.
           ·   If bit four is set (decimal 8) as a last resort the actual content of
               ‘application/octet-stream’ parts will be inspected, so that data which looks like
               plain text can be treated as such.  This mode is even more relaxed when data is to
               be displayed to the user or used as a message quote (data consumers which mangle
               data for display purposes, which includes masking of control characters, for
               example).

     mime-encoding
           The MIME ‘Content-Transfer-Encoding’ to use in outgoing text messages and message
           parts, where applicable.  (7-bit clean text messages are sent as-is, without a
           transfer encoding.)  Valid values are:

           ‘8bit’  (Or ‘8b’.)  8-bit transport effectively causes the raw data be passed through
                   unchanged, but may cause problems when transferring mail messages over
                   channels that are not ESMTP (RFC 1869) compliant.  Also, several input data
                   constructs are not allowed by the specifications and may cause a different
                   transfer-encoding to be used.
           ‘quoted-printable’
                   (Or ‘qp’.)  Quoted-printable encoding is 7-bit clean and has the property that
                   ASCII characters are passed through unchanged, so that an english message can
                   be read as-is; it is also acceptable for other single-byte locales that share
                   many characters with ASCII, like, e.g., ISO-8859-1.  The encoding will cause a
                   large overhead for messages in other character sets: e.g., it will require up
                   to twelve (12) bytes to encode a single UTF-8 character of four (4) bytes.  It
                   is the default encoding.
           ‘base64’
                   (Or ‘b64’.)  This encoding is 7-bit clean and will always be used for binary
                   data.  This encoding has a constant input:output ratio of 3:4, regardless of
                   the character set of the input data it will encode three bytes of input to
                   four bytes of output.  This transfer-encoding is not human readable without
                   performing a decoding step.

     mimetypes-load-control
           Can be used to control which of The mime.types files are loaded: if the letter ‘u’ is
           part of the option value, then the user's personal ~/.mime.types file will be loaded
           (if it exists); likewise the letter ‘s’ controls loading of the system wide /etc/
           mime.types; directives found in the user file take precedence, letter matching is
           case-insensitive.  If this variable is not set S-nail will try to load both files.
           Incorporation of the S-nail-built-in MIME types cannot be suppressed, but they will be
           matched last (the order can be listed via mimetype).

           More sources can be specified by using a different syntax: if the value string
           contains an equals sign ‘=’ then it is instead parsed as a comma-separated list of the
           described letters plus ‘f=FILENAME’ pairs; the given filenames will be expanded and
           loaded, and their content may use the extended syntax that is described in the section
           The mime.types files.  Directives found in such files always take precedence (are
           prepended to the MIME type cache).

     mta   To choose an alternate Mail-Transfer-Agent, set this option to either the full
           pathname of an executable (optionally prefixed with the protocol ‘file://’), or
           [Option]ally a SMTP a.k.a. SUBMISSION protocol URL, e.g., [v15-compat]

                 submissions://[user[:password]@]server[:port]

           ([no v15-compat]: ‘[smtp://]server[:port]’.)  The default has been chosen at compile
           time.  All supported data transfers are executed in child processes, which run
           asynchronously and without supervision unless either the sendwait or the verbose
           variable is set.  If such a child receives a TERM signal, it will abort and save the
           message to DEAD, if so configured.

           For a file-based MTA it may be necessary to set mta-argv0 in in order to choose the
           right target of a modern mailwrapper(8) environment.  It will be passed command line
           arguments from several possible sources: from the variable mta-arguments if set, from
           the command line if given and the variable expandargv allows their use.  Argument
           processing of the MTA will be terminated with a -- separator.

           The otherwise occurring implicit usage of the following MTA command line arguments can
           be disabled by setting the boolean variable mta-no-default-arguments (which will also
           disable passing -- to the MTA): -i (for not treating a line with only a dot ‘.’
           character as the end of input), -m (shall the variable metoo be set) and -v (if the
           verbose variable is set); in conjunction with the -r command line option S-nail will
           also (not) pass -f as well as possibly -F.

           [Option]ally S-nail can send mail over SMTP a.k.a. SUBMISSION network connections to a
           single defined smart host by setting this variable to a SMTP or SUBMISSION URL (see On
           URL syntax and credential lookup).  An authentication scheme can be specified via the
           variable chain smtp-auth.  Encrypted network connections are [Option]ally available,
           the section Encrypted network communication should give an overview and provide links
           to more information on this.  Note that with some mail providers it may be necessary
           to set the smtp-hostname variable in order to use a specific combination of from,
           hostname and mta.  S-nail also supports forwarding of all network traffic over a
           specified socks-proxy.  The following SMTP variants may be used:

           ·   The plain SMTP protocol (RFC 5321) that normally lives on the server port 25 and
               requires setting the smtp-use-starttls variable to enter a TLS encrypted session
               state.  Assign a value like [v15-compat] ‘smtp://[user[:password]@]server[:port]’
               ([no v15-compat] ‘smtp://server[:port]’) to choose this protocol.

           ·   The so-called SMTPS which is supposed to live on server port 465 and is
               automatically TLS secured.  Unfortunately it never became a standardized protocol
               and may thus not be supported by your hosts network service database – in fact the
               port number has already been reassigned to other protocols!

               SMTPS is nonetheless a commonly offered protocol and thus can be chosen by
               assigning a value like [v15-compat] ‘smtps://[user[:password]@]server[:port]’ ([no
               v15-compat] ‘smtps://server[:port]’); due to the mentioned problems it is usually
               necessary to explicitly specify the port as ‘:465’, however.

           ·   The SUBMISSION protocol (RFC 6409) lives on server port 587 and is identically to
               the SMTP protocol from S-nail's point of view; it requires setting
               smtp-use-starttls to enter a TLS secured session state; e.g., [v15-compat]
               ‘submission://[user[:password]@]server[:port]’.

           ·   The SUBMISSIONS protocol (RFC 8314) that lives on server port 465 and is TLS
               secured by default.  It can be chosen by assigning a value like [v15-compat]
               ‘submissions://[user[:password]@]server[:port]’.  Due to the problems mentioned
               for SMTPS above and the fact that SUBMISSIONS is new and a successor that lives on
               the same port as the historical engineering mismanagement named SMTPS, it is
               usually necessary to explicitly specify the port as ‘:465’.

     mta-arguments
           Arguments to pass through to a file-based mta can be given via this variable, which is
           parsed according to Shell-style argument quoting into an array of arguments, and which
           will be joined onto MTA options from other sources, and then passed individually to
           the MTA: ‘? wysh set mta-arguments='-t -X "/tmp/my log"'’.

     mta-no-default-arguments
           (Boolean) Unless this variable is set S-nail will pass some well known standard
           command line options to a file-based mta (Mail-Transfer-Agent), see there for more.

     mta-no-receiver-arguments
           (Boolean) By default a file-based mta will be passed all receiver addresses on the
           command line.  This variable can be set to suppress any such argument.

     mta-argv0
           Many systems use a so-called mailwrapper(8) environment to ensure compatibility with
           sendmail(1).  This works by inspecting the name that was used to invoke the mail
           delivery system.  If this variable is set then the mailwrapper (the program that is
           actually executed when calling the file-based mta) will treat its contents as that
           name.

     netrc-lookup-USER@HOST, netrc-lookup-HOST, netrc-lookup
           (Boolean)[v15-compat][Option] Used to control usage of the user's ~/.netrc file for
           lookup of account credentials, as documented in the section On URL syntax and
           credential lookup and for the command netrc; the section The .netrc file documents the
           file format.  Also see netrc-pipe.

     netrc-pipe
           [v15-compat][Option] When ~/.netrc is loaded (see netrc and netrc-lookup) then S-nail
           will read the output of a shell pipe instead of the user's ~/.netrc file if this
           variable is set (to the desired shell command).  This can be used to, e.g., store ~/
           .netrc in encrypted form: ‘? set netrc-pipe='gpg -qd ~/.netrc.pgp'’.

     newfolders
           [Option] If this variable has the value ‘maildir’, newly created local folders will be
           in Maildir instead of MBOX format.

     newmail
           Checks for new mail in the current folder each time the prompt is shown.  A Maildir
           folder must be re-scanned to determine if new mail has arrived.  If this variable is
           set to the special value ‘nopoll’ then a Maildir folder will not be rescanned
           completely, but only timestamp changes are detected.  Maildir folders are [Option]al.

     outfolder
           (Boolean) Unless specified as absolute pathnames, causes the filename given in the
           record variable and the sender-based filenames for the Copy and Save commands to be
           interpreted relative to the directory given in the folder variable rather than
           relative to the current directory.

     on-account-cleanup-ACCOUNT, on-account-cleanup
           Macro hook which will be called once an account is left, as the very last step before
           unrolling per-account localopts.  This hook is run even in case of fatal errors, and
           it is advisable to perform only absolutely necessary actions, like cleaning up
           alternates, for example.  The specialized form is used in favour of the generic one if
           found.

     on-compose-cleanup
           Macro hook which will be called after the message has been sent (or not, in case of
           failures), as the very last step before unrolling compose mode localopts.  This hook
           is run even in case of fatal errors, and it is advisable to perform only absolutely
           necessary actions, like cleaning up alternates, for example.

           For compose mode hooks that may affect the message content please see
           on-compose-enter, on-compose-leave, on-compose-splice.  [v15 behaviour may differ]
           This hook exists because alias, alternates, commandalias, shortcut, to name a few, are
           not covered by localopts: changes applied in compose mode will continue to be in
           effect thereafter.

     on-compose-enter, on-compose-leave
           Macro hooks which will be called once compose mode is entered, and after composing has
           been finished, but before a set message-inject-tail has been injected etc.,
           respectively.  localopts are enabled for these hooks, and changes on variables will be
           forgotten after the message has been sent.  on-compose-cleanup can be used to perform
           other necessary cleanup steps.

           The following (read-only) variables will be set temporarily during execution of the
           macros to represent respective message headers, to the empty string otherwise; most of
           them correspond to according virtual message headers that can be accessed via ~^, one
           of the COMMAND ESCAPES (also from within on-compose-splice hooks):

           mailx-command  The command that generates the message.
           mailx-subject  The subject.
           mailx-from     from.
           mailx-sender   sender.
           mailx-to, mailx-cc, mailx-bcc
                          The list of receiver addresses as a space-separated list.
           mailx-raw-to, mailx-raw-cc, mailx-raw-bcc
                          The list of receiver addresses before any mangling (due to, e.g.,
                          alternates, alias recipients-in-cc) as a space-separated list.
           mailx-orig-from
                          When replying, forwarding or resending, this will be set to the ‘From:’
                          of the given message.
           mailx-orig-to, mailx-orig-cc, mailx-orig-bcc
                          When replying, forwarding or resending, this will be set to the
                          receivers of the given message.

           Here is am example that injects a signature via message-inject-tail; instead using
           on-compose-splice to simply inject the file of desire via ~< or ~<! may be a better
           approach.

                 define t_ocl {
                   vput ! i cat ~/.mysig
                   if [ $? -eq 0 ]
                      vput vexpr message-inject-tail trim-end $i
                   end

                   # Alternatively
                   readctl create ~/.mysig
                   if [ $? -eq 0 ]
                     readall i
                     if [ $? -eq 0 ]
                       vput vexpr message-inject-tail trim-end $i
                     end
                     readctl remove ~/.mysig
                   end
                 }
                 set on-compose-leave=t_ocl

     on-compose-splice, on-compose-splice-shell
           These hooks run once the normal compose mode is finished, but before the
           on-compose-leave macro hook is called, the message-inject-tail is injected etc.  Both
           hooks will be executed in a subprocess, with their input and output connected to
           S-nail such that they can act as if they would be an interactive user.  The difference
           in between them is that the latter is a SHELL command, whereas the former is a normal
           S-nail macro, but which is restricted to a small set of commands (the verbose output
           of, e.g., list will indicate said capability).  localopts are enabled for these hooks
           (in the parent process), causing any setting to be forgotten after the message has
           been sent; on-compose-cleanup can be used to perform other cleanup as necessary.

           During execution of these hooks S-nail will temporarily forget whether it has been
           started in interactive mode, (a restricted set of) COMMAND ESCAPES will always be
           available, and for guaranteed reproducibilities sake escape and ifs will be set to
           their defaults.  The compose mode command ~^ has been especially designed for
           scriptability (via these hooks).  The first line the hook will read on its standard
           input is the protocol version of said command escape, currently “0 0 1”: backward
           incompatible protocol changes have to be expected.

           Care must be taken to avoid deadlocks and other false control flow: if both involved
           processes wait for more input to happen at the same time, or one does not expect more
           input but the other is stuck waiting for consumption of its output, etc.  There is no
           automatic synchronization of the hook: it will not be stopped automatically just
           because it, e.g., emits ‘~x’.  The hooks will however receive a termination signal if
           the parent enters an error condition.  [v15 behaviour may differ] Protection against
           and interaction with signals is not yet given; it is likely that in the future these
           scripts will be placed in an isolated session, which is signalled in its entirety as
           necessary.

                 define ocs_signature {
                   read version
                   echo '~< ~/.mysig' # '~<! fortune pathtofortunefile'
                 }
                 set on-compose-splice=ocs_signature

                 wysh set on-compose-splice-shell=$'\
                   read version;\
                   printf "hello $version!  Headers: ";\
                   echo \'~^header list\';\
                   read status result;\
                   echo "status=$status result=$result";\
                   '

                 define ocsm {
                   read version
                   echo Splice protocol version is $version
                   echo '~^h l'; read hl; vput vexpr es substring "${hl}" 0 1
                   if [ "$es" != 2 ]
                     echoerr 'Cannot read header list'; echo '~x'; xit
                   endif
                   if [ "$hl" @i!% ' cc' ]
                     echo '~^h i cc Diet is your <mirr.or>'; read es;\
                       vput vexpr es substring "${es}" 0 1
                     if [ "$es" != 2 ]
                       echoerr 'Cannot insert Cc: header'; echo '~x'
                       # (no xit, macro finishs anyway)
                     endif
                   endif
                 }
                 set on-compose-splice=ocsm

     on-resend-cleanup
           [v15 behaviour may differ] Identical to on-compose-cleanup, but is only triggered by
           resend.

     on-resend-enter
           [v15 behaviour may differ] Identical to on-compose-enter, but is only triggered by
           resend.

     page  (Boolean) If set, each message feed through the command given for pipe is followed by
           a formfeed character ‘\f’.

     password-USER@HOST, password-HOST, password
           [v15-compat] Variable chain that sets a password, which is used in case none has been
           given in the protocol and account-specific URL; as a last resort S-nail will ask for a
           password on the user's terminal if the authentication method requires a password.
           Specifying passwords in a startup file is generally a security risk; the file should
           be readable by the invoking user only.

     password-USER@HOST
           [no v15-compat] (see the chain above for [v15-compat]) Set the password for ‘USER’
           when connecting to ‘HOST’.  If no such variable is defined for a host, the user will
           be asked for a password on standard input.  Specifying passwords in a startup file is
           generally a security risk; the file should be readable by the invoking user only.

     piperaw
           (Boolean) Send messages to the pipe command without performing MIME and character set
           conversions.

     pipe-TYPE/SUBTYPE
           When a MIME message part of type ‘TYPE/SUBTYPE’ (case-insensitive) is displayed or
           quoted, its text is filtered through the value of this variable interpreted as a shell
           command.  Note that only parts which can be displayed inline as plain text (see
           copiousoutput) are displayed unless otherwise noted, other MIME parts will only be
           considered by and for the command mimeview.

           The special value commercial at ‘@’ forces interpretation of the message part as plain
           text, e.g., ‘set pipe-application/xml=@’ will henceforth display XML “as is”.  (The
           same could also be achieved by adding a MIME type marker with the mimetype command.
           And [Option]ally MIME type handlers may be defined via The Mailcap files — these
           directives, copiousoutput has already been used, should be referred to for further
           documentation.

           The commercial at ‘@’ can in fact be used as a trigger character to adjust usage and
           behaviour of a following shell command specification more thoroughly by appending more
           special characters which refer to further mailcap directives, e.g., the following
           hypothetical command specification could be used:

                 ? set pipe-X/Y='@!++=@vim ${MAILX_FILENAME_TEMPORARY}'

           ‘*’   The command produces plain text to be integrated in S-nails output:
                 copiousoutput.
           ‘#’   If set the handler will not be invoked when a message is to be quoted, but only
                 when it will be displayed: x-mailx-noquote.
           ‘&’   Run the command asynchronously, i.e., without blocking S-nail: x-mailx-async.
           ‘!’   The command must be run on an interactive terminal, S-nail will temporarily
                 release the terminal to it: needsterminal.
           ‘+’   Request creation of a zero-sized temporary file, the absolute pathname of which
                 will be made accessible via the environment variable MAILX_FILENAME_TEMPORARY:
                 x-mailx-tmpfile.  If given twice then the file will be unlinked automatically by
                 S-nail when the command loop is entered again at latest: x-mailx-tmpfile-unlink.
           ‘=’   Normally the MIME part content is passed to the handler via standard input; if
                 this flag is set then the data will instead be written into
                 MAILX_FILENAME_TEMPORARY (x-mailx-tmpfile-fill), the creation of which is
                 implied; note however that in order to cause deletion of the temporary file you
                 still have to use two plus signs ‘++’ explicitly!
           ‘@’   To avoid ambiguities with normal shell command content you can use another
                 commercial at to forcefully terminate interpretation of remaining characters.
                 (Any character not in this list will have the same effect.)

           Some information about the MIME part to be displayed is embedded into the environment
           of the shell command:

           MAILX_CONTENT            The MIME content-type of the part, if known, the empty string
                                    otherwise.
           MAILX_CONTENT_EVIDENCE   If mime-counter-evidence includes the carry-around-bit (2),
                                    then this will be set to the detected MIME content-type; not
                                    only then identical to MAILX_CONTENT otherwise.
           MAILX_EXTERNAL_BODY_URL  MIME parts of type ‘message/external-body access-type=url’
                                    will store the access URL in this variable, it is empty
                                    otherwise.  URL targets should not be activated
                                    automatically, without supervision.
           MAILX_FILENAME           The filename, if any is set, the empty string otherwise.
           MAILX_FILENAME_GENERATED
                                    A random string.
           MAILX_FILENAME_TEMPORARY
                                    If temporary file creation has been requested through the
                                    command prefix this variable will be set and contain the
                                    absolute pathname of the temporary file.

     pipe-EXTENSION
           This is identical to pipe-TYPE/SUBTYPE except that ‘EXTENSION’ (normalized to
           lowercase using character mappings of the ASCII charset) names a file extension, e.g.,
           ‘xhtml’.  Handlers registered using this method take precedence.

     pop3-auth-USER@HOST, pop3-auth-HOST, pop3-auth
           [Option][v15-compat] Variable chain that sets the POP3 authentication method.  The
           only possible value as of now is ‘plain’, which is thus the default.

     pop3-bulk-load-USER@HOST, pop3-bulk-load-HOST, pop3-bulk-load
           (Boolean)[Option] When accessing a POP3 server S-nail loads the headers of the
           messages, and only requests the message bodies on user request.  For the POP3 protocol
           this means that the message headers will be downloaded twice.  If this variable is set
           then S-nail will download only complete messages from the given POP3 server(s)
           instead.

     pop3-keepalive-USER@HOST, pop3-keepalive-HOST, pop3-keepalive
           [Option] POP3 servers close the connection after a period of inactivity; the standard
           requires this to be at least 10 minutes, but practical experience may vary.  Setting
           this variable to a numeric value greater than ‘0’ causes a ‘NOOP’ command to be sent
           each value seconds if no other operation is performed.

     pop3-no-apop-USER@HOST, pop3-no-apop-HOST, pop3-no-apop
           (Boolean)[Option] Unless this variable is set the ‘APOP’ authentication method will be
           used when connecting to a POP3 server that advertises support.  The advantage of
           ‘APOP’ is that the password is not sent in clear text over the wire and that only a
           single packet is sent for the user/password tuple.  Note that pop3-no-apop-HOST
           requires [v15-compat].

     pop3-use-starttls-USER@HOST, pop3-use-starttls-HOST, pop3-use-starttls
           (Boolean)[Option] Causes S-nail to issue a ‘STLS’ command to make an unencrypted POP3
           session TLS encrypted.  This functionality is not supported by all servers, and is not
           used if the session is already encrypted by the POP3S method.  Note that
           pop3-use-starttls-HOST requires [v15-compat].

     posix
           (Boolean) This flag enables POSIX mode, which changes behaviour of S-nail where that
           deviates from standardized behaviour.  It will be set implicitly before the Resource
           files are loaded if the environment variable POSIXLY_CORRECT is set, and adjusting any
           of those two will be reflected by the other one implicitly.  The following behaviour
           is covered and enforced by this mechanism:

           ·   In non-interactive mode, any error encountered while loading resource files during
               program startup will cause a program exit, whereas in interactive mode such errors
               will stop loading of the currently loaded (stack of) file(s, i.e., recursively).
               These exits can be circumvented on a per-command base by using ignerr, one of the
               Command modifiers, for each command which shall be allowed to fail.
           ·   alternates will replace the list of alternate addresses instead of appending to
               it.  In addition alternates will only be honoured for any sort of message reply,
               and for aliases.
           ·   The variable inserting COMMAND ESCAPES ~A, ~a, ~I and ~i will expand embedded
               character sequences ‘\t’ horizontal tabulator and ‘\n’ line feed.  [v15 behaviour
               may differ] For compatibility reasons this step will always be performed.
           ·   Upon changing the active file no summary of headers will be displayed even if
               header is set.
           ·   Setting ignoreeof implies the behaviour described by dot.
           ·   The variable keep is extended to cover any empty mailbox, not only empty primary
               system mailboxes: they will be removed when they are left in empty state
               otherwise.

     print-alternatives
           (Boolean) When a MIME message part of type ‘multipart/alternative’ is displayed and it
           contains a subpart of type ‘text/plain’, other parts are normally discarded.  Setting
           this variable causes all subparts to be displayed, just as if the surrounding part was
           of type ‘multipart/mixed’.

     prompt
           The string used as a prompt in interactive mode.  Whenever the variable is evaluated
           the value is treated as if specified within dollar-single-quotes (see Shell-style
           argument quoting).  This (post-assignment, i.e., second) expansion can be used to
           embed status information, for example ?, !, account or mailbox-display.

           In order to embed characters which should not be counted when calculating the visual
           width of the resulting string, enclose the characters of interest in a pair of reverse
           solidus escaped brackets: ‘\[\E[0m\]’; a slot for coloured prompts is also available
           with the [Option]al command colour.  Prompting may be prevented by setting this to the
           null string (a.k.a. ‘set noprompt’).

     prompt2
           This string is used for secondary prompts, but is otherwise identical to prompt.  The
           default is ‘.. ’.

     quiet
           (Boolean) Suppresses the printing of the version when first invoked.

     quote
           If set a reply message is started with the quoted original message, the lines of which
           are prefixed by the value of the variable indentprefix, taking into account
           quote-chars and quote-fold.  If set to the empty value, the quoted message will be
           preceded and followed by the expansions of the values of quote-inject-head and
           quote-inject-tail, respectively.  None of the headers of the quoted message is
           included in the quote if the value equals ‘noheading’, and only the headers selected
           by the ‘type’ headerpick selection are put above the message body for ‘headers’,
           whereas all headers and all MIME parts are included for ‘allheaders’.  Also see
           quote-as-attachment and ~Q, one of the COMMAND ESCAPES.

     quote-as-attachment
           (Boolean) Add the original message in its entirety as a ‘message/rfc822’ MIME
           attachment when replying to a message.  Note this works regardless of the setting of
           quote.

     quote-chars
           Can be set to a string consisting of non-whitespace ASCII characters which shall be
           treated as quotation leaders, the default being ‘>|}:’.

     quote-fold
           [Option] Can be set in addition to indentprefix, and creates a more fancy quotation in
           that leading quotation characters (quote-chars) are compressed and overlong lines are
           folded.  quote-fold can be set to either one, two or three (space separated) numeric
           values, which are interpreted as the maximum (goal) and the minimum line length,
           respectively, in a spirit rather equal to the fmt(1) program, but line- instead of
           paragraph-based.  The third value is used as the maximum line length instead of the
           first if no better break point can be found; it is ignored unless it is larger than
           the minimum and smaller than the maximum.  If not set explicitly the minimum will
           reflect the goal algorithmically.  The goal cannot be smaller than the length of
           indentprefix plus some additional pad; necessary adjustments take place silently.

     quote-inject-head, quote-inject-tail
           The strings to put before and after the text of a quoted message, respectively.  The
           former defaults to ‘%f wrote:\n\n’.  Special format directives will be expanded if
           possible, and if so configured the output will be folded according to quote-fold.
           Format specifiers in the given strings start with a percent sign ‘%’ and expand values
           of the original message, unless noted otherwise.  Note that names and addresses are
           not subject to the setting of showto.  Valid format specifiers are:

           ‘%%’    A plain percent sign.
           ‘%a’    The address(es) of the sender(s).
           ‘%d’    The date found in the ‘Date:’ header of the message when datefield is set (the
                   default), otherwise the date when the message was received.  Formatting can be
                   controlled by assigning a strftime(3) format string to datefield (and
                   datefield-markout-older).
           ‘%f’    The full name(s) (name and address, as given) of the sender(s).
           ‘%i’    The ‘Message-ID:’.
           ‘%n’    The real name(s) of the sender(s) if there is one and showname allows usage,
                   the address(es) otherwise.
           ‘%r’    The senders real name(s) if there is one, the address(es) otherwise.

     r-option-implicit
           (Boolean) Setting this option evaluates the contents of from (or, if that contains
           multiple addresses, sender) and passes the results onto the used (file-based) MTA as
           described for the -r option (empty argument case).

     recipients-in-cc
           (Boolean) When doing a reply, the original ‘From:’ and ‘To:’ are by default merged
           into the new ‘To:’.  If this variable is set, only the original ‘From:’ ends in the
           new ‘To:’, the rest is merged into ‘Cc:’.

     record
           Unless this variable is defined, no copies of outgoing mail will be saved.  If defined
           it gives the pathname, subject to the usual Filename transformations, of a folder
           where all new, replied-to or forwarded messages are saved: when saving to this folder
           fails the message is not sent, but instead saved to DEAD.  The standard defines that
           relative (fully expanded) paths are to be interpreted relative to the current
           directory (cwd), to force interpretation relative to folder outfolder needs to be set
           in addition.

     record-files
           (Boolean) If this variable is set the meaning of record will be extended to cover
           messages which target only file and pipe recipients (see expandaddr).  These address
           types will not appear in recipient lists unless add-file-recipients is also set.

     record-resent
           (Boolean) If this variable is set the meaning of record will be extended to also cover
           the resend and Resend commands.

     reply-in-same-charset
           (Boolean) If this variable is set S-nail first tries to use the same character set of
           the original message for replies.  If this fails, the mechanism described in Character
           sets is evaluated as usual.

     reply-strings
           Can be set to a comma-separated list of (case-insensitive according to ASCII rules)
           strings which shall be recognized in addition to the built-in strings as ‘Subject:’
           reply message indicators – built-in are ‘Re:’, which is mandated by RFC 5322, as well
           as the german ‘Aw:’, ‘Antw:’, and the ‘Wg:’ which often has been seen in the wild;
           I.e., the separating colon has to be specified explicitly.

     reply-to
           A list of addresses to put into the ‘Reply-To:’ field of the message header.  Members
           of this list are handled as if they were in the alternates list.

     replyto
           [Obsolete] Variant of reply-to.

     reply-to-honour
           Controls whether a ‘Reply-To:’ header is honoured when replying to a message via reply
           or Lreply.  This is a quadoption; if set without a value it defaults to “yes”.

     rfc822-body-from_
           (Boolean) This variable can be used to force displaying a so-called ‘From_’ line for
           messages that are embedded into an envelope mail via the ‘message/rfc822’ MIME
           mechanism, for more visual convenience.

     save  (Boolean) Enable saving of (partial) messages in DEAD upon interrupt or delivery
           error.

     screen
           The number of lines that represents a “screenful” of lines, used in headers summary
           display, from searching, message topline display and scrolling via z.  If this
           variable is not set S-nail falls back to a calculation based upon the detected
           terminal window size and the baud rate: the faster the terminal, the more will be
           shown.  Overall screen dimensions and pager usage is influenced by the environment
           variables COLUMNS and LINES and the variable crt.

     searchheaders
           (Boolean) Expand message-list specifiers in the form ‘/x:y’ to all messages containing
           the substring “y” in the header field ‘x’.  The string search is case insensitive.

     sendcharsets
           [Option] A comma-separated list of character set names that can be used in outgoing
           internet mail.  The value of the variable charset-8bit is automatically appended to
           this list of character sets.  If no character set conversion capabilities are compiled
           into S-nail then the only supported charset is ttycharset.  Also see
           sendcharsets-else-ttycharset and refer to the section Character sets for the complete
           picture of character set conversion in S-nail.

     sendcharsets-else-ttycharset
           (Boolean)[Option] If this variable is set, but sendcharsets is not, then S-nail acts
           as if sendcharsets had been set to the value of the variable ttycharset.  In effect
           this combination passes through the message data in the character set of the current
           locale encoding: therefore mail message text will be (assumed to be) in ISO-8859-1
           encoding when send from within a ISO-8859-1 locale, and in UTF-8 encoding when send
           from within an UTF-8 locale.

           The 8-bit fallback charset-8bit never comes into play as ttycharset is implicitly
           assumed to be 8-bit and capable to represent all files the user may specify (as is the
           case when no character set conversion support is available in S-nail and the only
           supported character set is ttycharset: Character sets).  This might be a problem for
           scripts which use the suggested ‘LC_ALL=C’ setting, since in this case the character
           set is US-ASCII by definition, so that it is better to also override ttycharset, then.

     sender
           An address that is put into the ‘Sender:’ field of outgoing messages, quoting RFC
           5322: the mailbox of the agent responsible for the actual transmission of the message.
           This field should normally not be used unless the from field contains more than one
           address, on which case it is required.  Dependent on the context this address is
           handled as if it were in the list of alternates.  Also see -r, r-option-implicit.

     sendmail
           [Obsolete] Predecessor of mta.

     sendmail-arguments
           [Obsolete] Predecessor of mta-arguments.

     sendmail-no-default-arguments
           [Obsolete](Boolean) Predecessor of mta-no-default-arguments.

     sendmail-progname
           [Obsolete] Predecessor of mta-argv0.

     sendwait
           (Boolean) When sending a message wait until the mta (including the built-in SMTP one)
           exits before accepting further commands.  Only with this variable set errors reported
           by the MTA will be recognizable!  If the MTA returns a non-zero exit status, the exit
           status of S-nail will also be non-zero.

     showlast
           (Boolean) This setting causes S-nail to start at the last message instead of the first
           one when opening a mail folder, as well as with from and headers.

     showname
           (Boolean) Causes S-nail to use the sender's real name instead of the plain address in
           the header field summary and in message specifications.

     showto
           (Boolean) Causes the recipient of the message to be shown in the header summary if the
           message was sent by the user.

     Sign  The value backing ~A, one of the COMMAND ESCAPES.  Also see message-inject-tail,
           on-compose-leave and on-compose-splice.

     sign  The value backing ~a, one of the COMMAND ESCAPES.  Also see message-inject-tail,
           on-compose-leave and on-compose-splice.

     signature
           [Obsolete] Please use on-compose-splice or on-compose-splice-shell or on-compose-leave
           and (if necessary) message-inject-tail instead!

     skipemptybody
           (Boolean) If an outgoing message does not contain any text in its first or only
           message part, do not send it but discard it silently (see also the command line option
           -E).

     smime-ca-dir, smime-ca-file
           [Option] Specify the location of trusted CA certificates in PEM (Privacy Enhanced
           Mail) for the purpose of verification of S/MIME signed messages.  tls-ca-dir documents
           the necessary preparation steps to use the former.  The set of CA certificates which
           are built into the TLS library can be explicitly turned off by setting
           smime-ca-no-defaults, and further fine-tuning is possible via smime-ca-flags.

     smime-ca-flags
           [Option] Can be used to fine-tune behaviour of the X509 CA certificate storage, and
           the certificate verification that is used.  The actual values and their meanings are
           documented for tls-ca-flags.

     smime-ca-no-defaults
           (Boolean)[Option] Do not load the default CA locations that are built into the used to
           TLS library to verify S/MIME signed messages.

     smime-cipher-USER@HOST, smime-cipher
           [Option] Specifies the cipher to use when generating S/MIME encrypted messages (for
           the specified account).  RFC 5751 mandates a default of ‘aes128’ (AES-128 CBC).
           Possible values are (case-insensitive and) in decreasing cipher strength: ‘aes256’
           (AES-256 CBC), ‘aes192’ (AES-192 CBC), ‘aes128’ (AES-128 CBC), ‘des3’ (DES EDE3 CBC,
           168 bits; default if ‘aes128’ is not available) and ‘des’ (DES CBC, 56 bits).

           The actually available cipher algorithms depend on the cryptographic library that
           S-nail uses.  [Option] Support for more cipher algorithms may be available through
           dynamic loading via, e.g., EVP_get_cipherbyname(3) (OpenSSL) if S-nail has been
           compiled to support this.

     smime-crl-dir
           [Option] Specifies a directory that contains files with CRLs in PEM format to use when
           verifying S/MIME messages.

     smime-crl-file
           [Option] Specifies a file that contains a CRL in PEM format to use when verifying
           S/MIME messages.

     smime-encrypt-USER@HOST
           [Option] If this variable is set, messages send to the given receiver are encrypted
           before sending.  The value of the variable must be set to the name of a file that
           contains a certificate in PEM format.

           If a message is sent to multiple recipients, each of them for whom a corresponding
           variable is set will receive an individually encrypted message; other recipients will
           continue to receive the message in plain text unless the smime-force-encryption
           variable is set.  It is recommended to sign encrypted messages, i.e., to also set the
           smime-sign variable.

     smime-force-encryption
           (Boolean)[Option] Causes S-nail to refuse sending unencrypted messages.

     smime-sign
           (Boolean)[Option] S/MIME sign outgoing messages with the user's private key and
           include the user's certificate as a MIME attachment.  Signing a message enables a
           recipient to verify that the sender used a valid certificate, that the email addresses
           in the certificate match those in the message header and that the message content has
           not been altered.  It does not change the message text, and people will be able to
           read the message as usual.  Also see smime-sign-cert, smime-sign-include-certs and
           smime-sign-digest.

     smime-sign-cert-USER@HOST, smime-sign-cert
           [Option] Points to a file in PEM format.  For the purpose of signing and decryption
           this file needs to contain the user's private key, followed by his certificate.

           For message signing ‘USER@HOST’ is always derived from the value of from (or, if that
           contains multiple addresses, sender).  For the purpose of encryption the recipient's
           public encryption key (certificate) is expected; the command certsave can be used to
           save certificates of signed messages (the section Signed and encrypted messages with
           S/MIME gives some details).  This mode of operation is usually driven by the
           specialized form.

           When decrypting messages the account is derived from the recipient fields (‘To:’ and
           ‘Cc:’) of the message, which are searched for addresses for which such a variable is
           set.  S-nail always uses the first address that matches, so if the same message is
           sent to more than one of the user's addresses using different encryption keys,
           decryption might fail.

           For signing and decryption purposes it is possible to use encrypted keys, and the
           pseudo-host(s) ‘USER@HOST.smime-cert-key’ for the private key (and
           ‘USER@HOST.smime-cert-cert’ for the certificate stored in the same file) will be used
           for performing any necessary password lookup, therefore the lookup can be automated
           via the mechanisms described in On URL syntax and credential lookup.  For example, the
           hypothetical address ‘bob@exam.ple’ could be driven with a private key / certificate
           pair path defined in smime-sign-cert-bob@exam.ple, and needed passwords would then be
           looked up via the pseudo hosts ‘bob@exam.ple.smime-cert-key’ (and
           ‘bob@exam.ple.smime-cert-cert’).  To include intermediate certificates, use
           smime-sign-include-certs.

     smime-sign-digest-USER@HOST, smime-sign-digest
           [Option] Specifies the message digestto use when signing S/MIME messages.  Please
           remember that for this use case ‘USER@HOST’ refers to the variable from (or, if that
           contains multiple addresses, sender).  The available algorithms depend on the used
           cryptographic library, but at least one usable builtin algorithm is ensured as a
           default.  If possible the standard RFC 5751 will be violated by using ‘SHA512’ instead
           of the mandated ‘SHA1’ due to security concerns.

           S-nail will try to add built-in support for the following message digests, names are
           case-insensitive: ‘BLAKE2b512’, ‘BLAKE2s256’, ‘SHA3-512’, ‘SHA3-384’, ‘SHA3-256’,
           ‘SHA3-224’, as well as the widely available ‘SHA512’, ‘SHA384’, ‘SHA256’, ‘SHA224’,
           and the proposed insecure ‘SHA1’ and ‘MD5’.  More digests may [Option]ally be
           available through dynamic loading via, e.g., the OpenSSL function
           EVP_get_digestbyname(3).

     smime-sign-include-certs-USER@HOST, smime-sign-include-certs
           [Option] If used, this is supposed to a consist of a comma-separated list of files,
           each of which containing a single certificate in PEM format to be included in the
           S/MIME message in addition to the smime-sign-cert certificate.  This can be used to
           include intermediate certificates of the certificate authority, in order to allow the
           receiver's S/MIME implementation to perform a verification of the entire certificate
           chain, starting from a local root certificate, over the intermediate certificates,
           down to the smime-sign-cert.  Even though top level certificates may also be included
           in the chain, they won't be used for the verification on the receiver's side.

           For the purpose of the mechanisms involved here, ‘USER@HOST’ refers to the content of
           the internal variable from (or, if that contains multiple addresses, sender).  The
           pseudo-host ‘USER@HOST.smime-include-certs’ will be used for performing password
           lookups for these certificates, shall they have been given one, therefore the lookup
           can be automated via the mechanisms described in On URL syntax and credential lookup.

     smime-sign-message-digest-USER@HOST, smime-sign-message-digest
           [Obsolete][Option] Predecessor(s) of smime-sign-digest.

     smtp  [Obsolete][Option] To use the built-in SMTP transport, specify a SMTP URL in mta.
           [v15 behaviour may differ] For compatibility reasons a set smtp is used in preference
           of mta.

     smtp-auth-USER@HOST, smtp-auth-HOST, smtp-auth
           [Option] Variable chain that controls the SMTP mta authentication method, possible
           values are ‘none’ ([no v15-compat] default), ‘plain’ ([v15-compat] default), ‘login’
           as well as the [Option]al methods ‘cram-md5’ and ‘gssapi’.  The ‘none’ method does not
           need any user credentials, ‘gssapi’ requires a user name and all other methods require
           a user name and a password.  See [v15-compat] mta, user and password ([no v15-compat]
           smtp-auth-password and smtp-auth-user).  Note that smtp-auth-HOST is [v15-compat].
           [no v15-compat]: Note for smtp-auth-USER@HOST: may override dependent on sender
           address in the variable from.

     smtp-auth-password
           [Option][no v15-compat] Sets the global fallback password for SMTP authentication.  If
           the authentication method requires a password, but neither smtp-auth-password nor a
           matching smtp-auth-password-USER@HOST can be found, S-nail will ask for a password on
           the user's terminal.

     smtp-auth-password-USER@HOST
           [no v15-compat] Overrides smtp-auth-password for specific values of sender addresses,
           dependent upon the variable from.

     smtp-auth-user
           [Option][no v15-compat] Sets the global fallback user name for SMTP authentication.
           If the authentication method requires a user name, but neither smtp-auth-user nor a
           matching smtp-auth-user-USER@HOST can be found, S-nail will ask for a user name on the
           user's terminal.

     smtp-auth-user-USER@HOST
           [no v15-compat] Overrides smtp-auth-user for specific values of sender addresses,
           dependent upon the variable from.

     smtp-hostname
           [Option][v15-compat] Normally S-nail uses the variable from to derive the necessary
           ‘USER@HOST’ information in order to issue a ‘MAIL FROM:<>’ SMTP mta command.  Setting
           smtp-hostname can be used to use the ‘USER’ from the SMTP account (mta or the user
           variable chain) and the ‘HOST’ from the content of this variable (or, if that is the
           empty string, hostname or the local hostname as a last resort).  This often allows
           using an address that is itself valid but hosted by a provider other than which (in
           from) is about to send the message.  Setting this variable also influences generated
           ‘Message-ID:’ and ‘Content-ID:’ header fields.  If the [Option]al IDNA support is
           available (see idna-disable) variable assignment is aborted when a necessary
           conversion fails.

     smtp-use-starttls-USER@HOST, smtp-use-starttls-HOST, smtp-use-starttls
           (Boolean)[Option] Causes S-nail to issue a ‘STARTTLS’ command to make an SMTP mta
           session TLS encrypted, i.e., to enable transport layer security.

     socks-proxy-USER@HOST, socks-proxy-HOST, socks-proxy
           [Option] If this is set to the hostname (SOCKS URL) of a SOCKS5 server then S-nail
           will proxy all of its network activities through it.  This can be used to proxy SMTP,
           POP3 etc. network traffic through the Tor anonymizer, for example.  The following
           would create a local SOCKS proxy on port 10000 that forwards to the machine ‘HOST’,
           and from which the network traffic is actually instantiated:

                 # Create local proxy server in terminal 1 forwarding to HOST
                 $ ssh -D 10000 USER@HOST
                 # Then, start a client that uses it in terminal 2
                 $ s-nail -Ssocks-proxy-USER@HOST=localhost:10000

     spam-interface
           [Option] In order to use any of the spam-related commands (like, e.g., spamrate) the
           desired spam interface must be defined by setting this variable.  Please refer to the
           manual section Handling spam for the complete picture of spam handling in S-nail.  All
           or none of the following interfaces may be available:

           ‘spamc’   Interaction with spamc(1) from the spamassassin(1)
                     (http://spamassassin.apache.org SpamAssassin) suite.  Different to the
                     generic filter interface S-nail will automatically add the correct arguments
                     for a given command and has the necessary knowledge to parse the program's
                     output.  A default value for spamc-command will have been compiled into the
                     S-nail binary if spamc(1) has been found in PATH during compilation.  Shall
                     it be necessary to define a specific connection type (rather than using a
                     configuration file for that), the variable spamc-arguments can be used as
                     in, e.g., ‘-d server.example.com -p 783’.  It is also possible to specify a
                     per-user configuration via spamc-user.  Note that this interface does not
                     inspect the ‘is-spam’ flag of a message for the command spamforget.

           ‘filter’  generic spam filter support via freely configurable hooks.  This interface
                     is meant for programs like bogofilter(1) and requires according behaviour in
                     respect to the hooks' exit status for at least the command spamrate (‘0’
                     meaning a message is spam, ‘1’ for non-spam, ‘2’ for unsure and any other
                     return value indicating a hard error); since the hooks can include shell
                     code snippets diverting behaviour can be intercepted as necessary.  The
                     hooks are spamfilter-ham, spamfilter-noham, spamfilter-nospam,
                     spamfilter-rate and spamfilter-spam; the manual section Handling spam
                     contains examples for some programs.  The process environment of the hooks
                     will have the variable MAILX_FILENAME_GENERATED set.  Note that spam score
                     support for spamrate is not supported unless the [Option]tional regular
                     expression support is available and the spamfilter-rate-scanscore variable
                     is set.

     spam-maxsize
           [Option] Messages that exceed this size will not be passed through to the configured
           spam-interface.  If unset or 0, the default of 420000 bytes is used.

     spamc-command
           [Option] The path to the spamc(1) program for the ‘spamc’ spam-interface.  Note that
           the path is not expanded, but used “as is”.  A fallback path will have been compiled
           into the S-nail binary if the executable had been found during compilation.

     spamc-arguments
           [Option] Even though S-nail deals with most arguments for the ‘spamc’ spam-interface
           automatically, it may at least sometimes be desirable to specify connection-related
           ones via this variable, e.g., ‘-d server.example.com -p 783’.

     spamc-user
           [Option] Specify a username for per-user configuration files for the ‘spamc’
           spam-interface.  If this is set to the empty string then S-nail will use the name of
           the current user.

     spamfilter-ham, spamfilter-noham, spamfilter-nospam, spamfilter-rate, spamfilter-spam
           [Option] Command and argument hooks for the ‘filter’ spam-interface.  The manual
           section Handling spam contains examples for some programs.

     spamfilter-rate-scanscore
           [Option] Because of the generic nature of the ‘filter’ spam-interface spam scores are
           not supported for it by default, but if the [Option]nal regular expression support is
           available then setting this variable can be used to overcome this restriction.  It is
           interpreted as follows: first a number (digits) is parsed that must be followed by a
           semicolon ‘;’ and an extended regular expression.  Then the latter is used to parse
           the first output line of the spamfilter-rate hook, and, in case the evaluation is
           successful, the group that has been specified via the number is interpreted as a
           floating point scan score.

     ssl-ca-dir-USER@HOST, ssl-ca-dir-HOST, ssl-ca-dir, ssl-ca-file-USER@HOST, ssl-ca-file-HOST,
           ssl-ca-file
           [Obsolete][Option] Predecessors of tls-ca-file, tls-ca-dir.

     ssl-ca-flags-USER@HOST, ssl-ca-flags-HOST, ssl-ca-flags
           [Obsolete][Option] Predecessor of tls-ca-flags.

     ssl-ca-no-defaults-USER@HOST, ssl-ca-no-defaults-HOST, ssl-ca-no-defaults
           [Obsolete](Boolean)[Option] Predecessor of tls-ca-no-defaults.

     ssl-cert-USER@HOST, ssl-cert-HOST, ssl-cert
           [Obsolete][Option] Please use the Certificate slot of tls-config-pairs.

     ssl-cipher-list-USER@HOST, ssl-cipher-list-HOST, ssl-cipher-list
           [Obsolete][Option] Please use the CipherString slot of tls-config-pairs.

     ssl-config-file
           [Obsolete][Option] Predecessor of tls-config-file.

     ssl-config-module-USER@HOST, ssl-config-module-HOST, ssl-config-module
           [Obsolete][Option] Predecessor of tls-config-module.

     ssl-config-pairs-USER@HOST, ssl-config-pairs-HOST, ssl-config-pairs
           [Obsolete][Option] Predecessor of tls-config-pairs.

     ssl-crl-dir, ssl-crl-file
           [Obsolete][Option] Predecessors of tls-crl-dir, tls-crl-file.

     ssl-curves-USER@HOST, ssl-curves-HOST, ssl-curves
           [Obsolete][Option] Please use the Curves slot of tls-config-pairs.

     ssl-features
           [Obsolete][Option](Read-only) Predecessor of tls-features.

     ssl-key-USER@HOST, ssl-key-HOST, ssl-key
           [Obsolete][Option] Please use the PrivateKey slot of tls-config-pairs.

     ssl-method-USER@HOST, ssl-method-HOST, ssl-method
           [Obsolete][Option] Please use the Protocol slot of tls-config-pairs.

     ssl-protocol-USER@HOST, ssl-protocol-HOST, ssl-protocol
           [Obsolete][Option] Please use the Protocol slot of tls-config-pairs.

     ssl-rand-file
           [Obsolete][Option] Predecessor of tls-rand-file.

     ssl-verify-USER@HOST, ssl-verify-HOST, ssl-verify
           [Obsolete][Option] Predecessor of tls-verify.

     stealthmua
           If only set without an assigned value, then this setting inhibits the generation of
           the ‘Message-ID:’, ‘Content-ID:’ and ‘User-Agent:’ header fields that include obvious
           references to S-nail.  There are two pitfalls associated with this: First, the message
           id of outgoing messages is not known anymore.  Second, an expert may still use the
           remaining information in the header to track down the originating mail user agent.  If
           set to the value ‘noagent’, then the mentioned ‘Message-ID:’ and ‘Content-ID:’
           suppression does not occur.

     system-mailrc
           (Read-only) The compiled in path of the system wide initialization file one of the
           Resource files: /etc/s-nail.rc.

     termcap
           ([Option]) This specifies a comma-separated list of Terminal Information Library
           (libterminfo, -lterminfo) and/or Termcap Access Library (libtermcap, -ltermcap)
           capabilities (see On terminal control and line editor, escape commas with reverse
           solidus) to be used to overwrite or define entries.  Note this variable will only be
           queried once at program startup and can thus only be specified in resource files or on
           the command line.

           String capabilities form ‘cap=value’ pairs and are expected unless noted otherwise.
           Numerics have to be notated as ‘cap#number’ where the number is expected in normal
           decimal notation.  Finally, booleans do not have any value but indicate a true or
           false state simply by being defined or not; this indeed means that S-nail does not
           support undefining an existing boolean.  String capability values will undergo some
           expansions before use: for one notations like ‘^LETTER’ stand for ‘control-LETTER’,
           and for clarification purposes ‘\E’ can be used to specify ‘escape’ (the control
           notation ‘^[’ could lead to misreadings when a left bracket follows, which it does for
           the standard CSI sequence); finally three letter octal sequences, as in ‘\061’, are
           supported.  To specify that a terminal supports 256-colours, and to define sequences
           that home the cursor and produce an audible bell, one might write:

                 ? set termcap='Co#256,home=\E[H,bel=^G'

           The following terminal capabilities are or may be meaningful for the operation of the
           built-in line editor or S-nail in general:

           colors or Co
                max_colors: numeric capability specifying the maximum number of colours.  Note
                that S-nail does not actually care about the terminal beside that, but always
                emits ANSI / ISO 6429 escape sequences.
           rmcup or te / smcup or ti
                exit_ca_mode and enter_ca_mode, respectively: exit and enter the alternative
                screen ca-mode, effectively turning S-nail into a fullscreen application.  This
                must be enabled explicitly by setting termcap-ca-mode.
           smkx or ks / rmkx or ke
                keypad_xmit and keypad_local, respectively: enable and disable the keypad.  This
                is always enabled if available, because it seems even keyboards without keypads
                generate other key codes for, e.g., cursor keys in that case, and only if enabled
                we see the codes that we are interested in.
           ed or cd
                clr_eos: clear the screen.
           clear or cl
                clear_screen: clear the screen and home cursor.  (Will be simulated via ho plus
                cd.)
           home or ho
                cursor_home: home cursor.
           el or ce
                clr_eol: clear to the end of line.  (Will be simulated via ch plus repetitions of
                space characters.)
           hpa or ch
                column_address: move the cursor (to the given column parameter) in the current
                row.  (Will be simulated via cr plus nd.)
           cr   carriage_return: move to the first column in the current row.  The default built-
                in fallback is ‘\r’.
           cub1 or le
                cursor_left: move the cursor left one space (non-destructively).  The default
                built-in fallback is ‘\b’.
           cuf1 or nd
                cursor_right: move the cursor right one space (non-destructively).  The default
                built-in fallback is ‘\E[C’, which is used by most terminals.  Less often occur
                ‘\EC’ and ‘\EOC’.

           Many more capabilities which describe key-sequences are documented for bind.

     termcap-ca-mode
           [Option] Allow usage of the exit_ca_mode and enter_ca_mode terminal capabilities, see
           termcap.  Note this variable will only be queried once at program startup and can thus
           only be specified in resource files or on the command line.

     termcap-disable
           [Option] Disable any interaction with a terminal control library.  If set only some
           generic fallback built-ins and possibly the content of termcap describe the terminal
           to S-nail.  Note this variable will only be queried once at program startup and can
           thus only be specified in resource files or on the command line.

     tls-ca-dir-USER@HOST, tls-ca-dir-HOST, tls-ca-dir, tls-ca-file-USER@HOST, tls-ca-file-HOST,
           tls-ca-file
           [Option] Directory and file, respectively, for pools of trusted CA certificates in PEM
           (Privacy Enhanced Mail) format, for the purpose of verification of TLS server
           certificates.  Concurrent use is possible, the file is loaded once needed first, the
           directory lookup is performed anew as a last resort whenever necessary.  The CA
           certificate pool built into the TLS library can be disabled via tls-ca-no-defaults,
           further fine-tuning is possible via tls-ca-flags.  Note the directory search variant
           requires the certificate files to adhere special filename conventions, please see
           SSL_CTX_load_verify_locations(3) and verify(1) (or c_rehash(1)).

     tls-ca-flags-USER@HOST, tls-ca-flags-HOST, tls-ca-flags
           [Option] Can be used to fine-tune behaviour of the X509 CA certificate storage, and
           the certificate verification that is used (also see tls-verify).  The value is
           expected to consist of a comma-separated list of configuration directives, with any
           intervening whitespace being ignored.  The directives directly map to flags that can
           be passed to X509_STORE_set_flags(3), which are usually defined in a file
           openssl/x509_vfy.h, and the availability of which depends on the used TLS library
           version: a directive without mapping is ignored (error log subject to debug).
           Directives currently understood (case-insensitively) include:

           no-alt-chains
                 If the initial chain is not trusted, do not attempt to build an alternative
                 chain.  Setting this flag will make OpenSSL certificate verification match that
                 of older OpenSSL versions, before automatic building and checking of alternative
                 chains has been implemented; also see trusted-first.
           no-check-time
                 Do not check certificate/CRL validity against current time.
           partial-chain
                 By default partial, incomplete chains which cannot be verified up to the chain
                 top, a self-signed root certificate, will not verify.  With this flag set, a
                 chain succeeds to verify if at least one signing certificate of the chain is in
                 any of the configured trusted stores of CA certificates.  The OpenSSL manual
                 page SSL_CTX_load_verify_locations(3) gives some advise how to manage your own
                 trusted store of CA certificates.
           strict
                 Disable workarounds for broken certificates.
           trusted-first
                 Try building a chain using issuers in the trusted store first to avoid problems
                 with server-sent legacy intermediate certificates.  Newer versions of OpenSSL
                 support alternative chain checking and enable it by default, resulting in the
                 same behaviour; also see no-alt-chains.

     tls-ca-no-defaults-USER@HOST, tls-ca-no-defaults-HOST, tls-ca-no-defaults
           (Boolean)[Option] Do not load the default CA locations that are built into the used to
           TLS library to verify TLS server certificates.

     tls-config-file
           [Option] If this variable is set CONF_modules_load_file(3) (if announced via
           ‘+modules-load-file’ in tls-features) is used to allow resource file based
           configuration of the TLS library.  This happens once the library is used first, which
           may also be early during startup (logged with verbose)!  If a non-empty value is given
           then the given file, after performing Filename transformations, will be used instead
           of the TLS libraries global default, and it is an error if the file cannot be loaded.
           The application name will always be passed as ‘s-nail’.  Some TLS libraries support
           application-specific configuration via resource files loaded like this, please see
           tls-config-module.

     tls-config-module-USER@HOST, tls-config-module-HOST, tls-config-module
           [Option] If file based application-specific configuration via tls-config-file is
           available, announced as ‘+ctx-config’ by tls-features, indicating availability of
           SSL_CTX_config(3), then, it becomes possible to use a central TLS configuration file
           for all programs, including s-nail, e.g.:

                 # Register a configuration section for s-nail
                 s-nail = mailx_master
                 # The top configuration section creates a relation
                 # in between dynamic SSL configuration and an actual
                 # program specific configuration section
                 [mailx_master]
                 ssl_conf = mailx_tls_config
                 # Well that actual program specific configuration section
                 # now can map individual tls-config-module names to sections,
                 # e.g., tls-config-module=account_xy
                 [mailx_tls_config]
                 account_xy = mailx_account_xy
                 account_yz = mailx_account_yz
                 [mailx_account_xy]
                 MinProtocol = TLSv1.2
                 Curves=P-521
                 [mailx_account_yz]
                 CipherString = TLSv1.2:!aNULL:!eNULL:
                 MinProtocol = TLSv1.1
                 Options = Bugs

     tls-config-pairs-USER@HOST, tls-config-pairs-HOST, tls-config-pairs
           [Option] The value of this variable chain will be interpreted as a comma-separated
           list of directive/value pairs.  Directives and values need to be separated by equals
           signs ‘=’, any whitespace surrounding pair members is removed.  Keys are (usually)
           case-insensitive.  Different to when placing these pairs in a tls-config-module
           section of a tls-config-file, commas ‘,’ need to be escaped with a reverse solidus ‘\’
           when included in pairs; also different: if the equals sign ‘=’ is preceded with an
           asterisk ‘*’ Filename transformations will be performed on the value; it is an error
           if these fail.  Unless proper support is announced by tls-features (‘+conf-ctx’) only
           the keys below are supported, otherwise the pairs will be used directly as arguments
           to the function SSL_CONF_cmd(3).

           Certificate   Filename of a TLS client certificate (chain) required by some servers.
                         Fallback support via SSL_CTX_use_certificate_chain_file(3).  Filename
                         transformations are performed.  Note: if you use this you need to
                         specify the private key via PrivateKey, tls-key will not be used!
           CipherString  A list of ciphers for TLS connections, see ciphers(1).  By default no
                         list of ciphers is set, resulting in a Protocol-specific list of ciphers
                         (the protocol standards define lists of acceptable ciphers; possibly
                         cramped by the used TLS library).  Fallback support via
                         SSL_CTX_set_cipher_list(3).
           Ciphersuites  A list of ciphers used for TLSv1.3 connections, see ciphers(1).  These
                         will be joined onto the list of ciphers from CipherString.  Available if
                         tls-features announces ‘+ctx-set-ciphersuites’, as necessary via
                         SSL_CTX_set_ciphersuites(3).
           Curves        A list of supported elliptic curves, if applicable.  By default no
                         curves are set.  Fallback support via SSL_CTX_set1_curves_list(3), if
                         available.
           MaxProtocol, MinProtocol
                         The maximum and minimum supported TLS versions, respectively.  Available
                         if tls-features announces ‘+ctx-set-maxmin-proto’, as necessary via
                         SSL_CTX_set_max_proto_version(3) and SSL_CTX_set_min_proto_version(3);
                         these fallbacks use an internal parser which understands the strings
                         ‘SSLv3’, ‘TLSv1’, ‘TLSv1.1’, ‘TLSv1.2’, ‘TLSv1.3’, and the special value
                         ‘None’, which disables the given limit.
           Options       Various flags to set.  Fallback via SSL_CTX_set_options(3), in which
                         case any other value but (exactly) ‘Bugs’ results in an error.
           PrivateKey    Filename of the private key in PEM format of a TLS client certificate.
                         If unset, the name of the certificate file is used.  Filename
                         transformations are performed.  Fallback via
                         SSL_CTX_use_PrivateKey_file(3).  Note: if you use this you need to
                         specify the certificate (chain) via Certificate, tls-cert will not be
                         used!
           Protocol      The used TLS protocol.  If tls-features announces ‘+conf-ctx’ or
                         ‘ctx-set-maxmin-proto’ then using MaxProtocol and MinProtocol is
                         preferable.  Fallback is SSL_CTX_set_options(3), driven via an internal
                         parser which understands the strings ‘SSLv3’, ‘TLSv1’, ‘TLSv1.1’,
                         ‘TLSv1.2’, ‘TLSv1.3’, and the special value ‘ALL’.  Multiple protocols
                         may be given as a comma-separated list, any whitespace is ignored, an
                         optional plus sign ‘+’ prefix enables, a hyphen-minus ‘-’ prefix
                         disables a protocol, so that ‘-ALL, TLSv1.2’ enables only the TLSv1.2
                         protocol.

     tls-crl-dir, tls-crl-file
           [Option] Specify a directory / a file, respectively, that contains a CRL in PEM format
           to use when verifying TLS server certificates.

     tls-features
           [Option](Read-only) This expands to a comma separated list of the TLS library identity
           and optional SSL library features.  Currently supported identities are ‘libressl’
           (LibreSSL) , ‘libssl-0x10100’ (OpenSSL v1.1.x series) and ‘libssl-0x10000’ (elder
           OpenSSL series, other clones).  Optional features are preceded with a plus sign ‘+’
           when available, and with a hyphen-minus ‘-’ otherwise.

           Currently known features are ‘modules-load-file’ (tls-config-file), ‘conf-ctx’
           (tls-config-pairs), ‘ctx-config’ (tls-config-module), ‘ctx-set-maxmin-proto’
           (tls-config-pairs), ‘ctx-set-ciphersuites’ (the Ciphersuites slot of
           tls-config-pairs).

     tls-fingerprint-USER@HOST, tls-fingerprint-HOST, tls-fingerprint
           [Option] It is possible to replace the verification of the connection peer certificate
           against the entire local pool of CAs (for more see Encrypted network communication)
           with the comparison against a precalculated certificate message digest, the so-called
           fingerprint, to be specified as the used tls-fingerprint-digest.  This fingerprint can
           be calculated with, e.g., ‘tls fingerprint HOST’.

     tls-fingerprint-digest-USER@HOST, tls-fingerprint-digest-HOST, tls-fingerprint-digest
           [Option] The message digest to be used when creating TLS certificate fingerprints, the
           defaults, if available, in test order, being ‘BLAKE2s256’, ‘SHA256’.  For the complete
           list of digest algorithms refer to smime-sign-digest.

     tls-rand-file
           [Option] Gives the filename to a file with random entropy data, see RAND_load_file(3).
           If this variable is not set, or set to the empty string, or if the Filename
           transformations fail, then RAND_file_name(3) will be used to create the filename.  If
           the SSL PRNG was seeded successfully The file will be updated (RAND_write_file(3)) if
           and only if seeding and buffer stirring succeeds.

     tls-verify-USER@HOST, tls-verify-HOST, tls-verify
           [Option] Variable chain that sets the action to be performed if an error occurs during
           TLS server certificate validation against the specified or default trust stores
           tls-ca-dir, tls-ca-file, or the TLS library built-in defaults (unless usage disallowed
           via tls-ca-no-defaults), and as fine-tuned via tls-ca-flags.  Valid (case-insensitive)
           values are ‘strict’ (fail and close connection immediately), ‘ask’ (ask whether to
           continue on standard input), ‘warn’ (show a warning and continue), ‘ignore’ (do not
           perform validation).  The default is ‘ask’.

     toplines
           If defined, gives the number of lines of a message to be displayed with the command
           top; if unset, the first five lines are printed, if set to 0 the variable screen is
           inspected.  If the value is negative then its absolute value will be used for unsigned
           right shifting (see vexpr) the screen height.

     topsqueeze
           (Boolean) If set then the top command series will strip adjacent empty lines and
           quotations.

     ttycharset
           The character set of the terminal S-nail operates on, and the one and only supported
           character set that S-nail can use if no character set conversion capabilities have
           been compiled into it, in which case it defaults to ISO-8859-1.  Otherwise it defaults
           to UTF-8.  Sufficient locale support provided the default will be preferably deduced
           from the locale environment if that is set (e.g., LC_CTYPE, see there for more);
           runtime locale changes will be reflected by ttycharset except during the program
           startup phase and if -S had been used to freeze the given value.  Refer to the section
           Character sets for the complete picture about character sets.

     typescript-mode
           (Boolean) A special multiplex variable that disables all variables and settings which
           result in behaviour that interferes with running S-nail in script(1), e.g., it sets
           colour-disable, line-editor-disable and (before startup completed only)
           termcap-disable.  Unsetting it does not restore the former state of the covered
           settings.

     umask
           For a safe-by-default policy the process file mode creation mask umask(2) will be set
           to ‘0077’ on program startup by default.  Child processes inherit the file mode
           creation mask of their parent, and by setting this variable to an empty value no
           change will be applied, and the inherited value will be used.  Otherwise the given
           value will be made the new file mode creation mask.

     user-HOST, user
           [v15-compat] Variable chain that sets a global fallback user name, which is used in
           case none has been given in the protocol and account-specific URL.  This variable
           defaults to the name of the user who runs S-nail.

     v15-compat
           (Boolean) Setting this enables upward compatibility with S-nail version 15.0 in
           respect to which configuration options are available and how they are handled.  This
           manual uses [v15-compat] and [no v15-compat] to refer to the new and the old way of
           doing things, respectively.

     verbose
           (Boolean) This setting, also controllable via the command line option -v, causes
           S-nail to be more verbose, e.g., it will display obsoletion warnings and TLS
           certificate chains.  Even though marked (Boolean) this option may be set twice in
           order to increase the level of verbosity even more, in which case even details of the
           actual message delivery and protocol conversations are shown.  A single noverbose is
           sufficient to disable verbosity as such.

     version, version-date, version-hexnum, version-major, version-minor, version-update
           (Read-only) S-nail version information: the first variable is a string with the
           complete version identification, the second the release date in ISO 8601 notation
           without time.  The third is a 32-bit hexadecimal number with the upper 8 bits storing
           the major, followed by the minor and update version numbers which occupy 12 bits each.
           The latter three variables contain only decimal digits: the major, minor and update
           version numbers.  The output of the command version will include this information.

     writebackedited
           If this variable is set messages modified using the edit or visual commands are
           written back to the current folder when it is quit; it is only honoured for writable
           folders in MBOX format, though.  Note that the editor will be pointed to the raw
           message content in that case, i.e., neither MIME decoding nor decryption will have
           been performed, and proper RFC 4155 ‘From_’ quoting of newly added or edited content
           is also left as an exercise to the user.

ENVIRONMENT

     The term “environment variable” should be considered an indication that these variables are
     either standardized as vivid parts of process environments, or that they are commonly found
     in there.  The process environment is inherited from the sh(1) once S-nail is started, and
     unless otherwise explicitly noted handling of the following variables transparently
     integrates into that of the INTERNAL VARIABLES from S-nail's point of view.  This means
     that, e.g., they can be managed via set and unset, causing automatic program environment
     updates (to be inherited by newly created child processes).

     In order to integrate other environment variables equally they need to be imported (linked)
     with the command environ.  This command can also be used to set and unset non-integrated
     environment variables from scratch, sufficient system support provided.  The following
     example, applicable to a POSIX shell, sets the COLUMNS environment variable for S-nail only,
     and beforehand exports the EDITOR in order to affect any further processing in the running
     shell:

           $ EDITOR="vim -u ${HOME}/.vimrc"
           $ export EDITOR
           $ COLUMNS=80 s-nail -R

     COLUMNS
           The user's preferred width in column positions for the terminal screen or window.
           Queried and used once on program startup, actively managed for child processes and the
           MLE (see On terminal control and line editor) in interactive mode thereafter.  Ignored
           in non-interactive mode, which always uses 80 columns, unless in -# batch mode.

     DEAD  The name of the (mailbox) file to use for saving aborted messages if save is set; this
           defaults to ~/dead.letter.  If the variable debug is set no output will be generated,
           otherwise the contents of the file will be replaced.

     EDITOR
           Pathname of the text editor to use for the edit command and ~e (see COMMAND ESCAPES);
           VISUAL is used for a more display oriented editor.

     HOME  The user's home directory.  This variable is only used when it resides in the process
           environment.  The calling user's home directory will be used instead if this directory
           does not exist, is not accessible or cannot be read; it will always be used for the
           root user.  (No test for being writable is performed to allow usage by non-privileged
           users within read-only jails, but dependent on the variable settings this directory is
           a default write target, e.g. for DEAD, MBOX and more.)

     LC_ALL, LC_CTYPE, LANG
           [Option] The (names in lookup order of the) locale(7) (and / or see setlocale(3))
           which indicates the used Character sets.  Runtime changes trigger automatic updates of
           the entire locale system, which includes updating ttycharset (except during startup if
           the variable has been frozen via -S).

     LINES
           The user's preferred number of lines on a page or the vertical screen or window size
           in lines.  Queried and used once on program startup, actively managed for child
           processes in interactive mode thereafter.  Ignored in non-interactive mode, which
           always uses 24 lines, unless in
           batch mode.

     LISTER
           Pathname of the directory lister to use in the folders command when operating on local
           mailboxes.  Default is ls(1) (path search through SHELL).

     LOGNAME
           Upon startup S-nail will actively ensure that this variable refers to the name of the
           user who runs S-nail, in order to be able to pass a verified name to any newly created
           child process.

     MAIL  Is used as the user's primary system mailbox unless inbox is set.  This is assumed to
           be an absolute pathname.  If this environmental fallback is also not set, a built-in
           compile-time default is used.

     MAILCAPS
           [Option] Overrides the default path search for The Mailcap files, which is defined in
           the standard RFC 1524 as ‘~/.mailcap:/etc/mailcap:/usr/etc/mailcap:
           /usr/local/etc/mailcap’.  (S-nail makes it a configuration option, however.)  Note
           this is not a search path, but a path search.

     MAILRC
           Is used as a startup file instead of ~/.mailrc if set.  In order to avoid side-effects
           from configuration files scripts should either set this variable to /dev/null or the
           -: command line option should be used.

     MAILX_NO_SYSTEM_RC
           If this variable is set then reading of /etc/s-nail.rc (a.k.a. system-mailrc) at
           startup is inhibited, i.e., the same effect is achieved as if S-nail had been started
           up with the option -: (and according argument) or -n.  This variable is only used when
           it resides in the process environment.

     MBOX  The name of the user's secondary mailbox file.  A logical subset of the special
           Filename transformations (also see file) are supported.  The default is ~/mbox.
           Traditionally this MBOX is used as the file to save messages from the primary system
           mailbox that have been read.  Also see Message states.

     NETRC
           [v15-compat][Option] This variable overrides the default location of the user's ~/
           .netrc file.

     PAGER
           Pathname of the program to use for backing the command more, and when the crt variable
           enforces usage of a pager for output.  The default paginator is more(1) (path search
           through SHELL).

           S-nail inspects the contents of this variable: if its contains the string “less” then
           a non-existing environment variable LESS will be set to ‘Ri’, likewise for “lv” LV
           will optionally be set to ‘-c’.  Alse see colour-pager.

     PATH  A colon-separated list of directories that is searched by the shell when looking for
           commands, e.g., ‘/bin:/usr/bin:/usr/local/bin’.

     POSIXLY_CORRECT
           This variable is automatically looked for upon startup, see posix for more.

     SHELL
           The shell to use for the commands !, shell, the ~! COMMAND ESCAPES and when starting
           subprocesses.  A default shell is used if this environment variable is not defined.

     SOURCE_DATE_EPOCH
           Specifies a time in seconds since the Unix epoch (1970-01-01) to be used in place of
           the current time.  This variable is looked up upon program startup, and its existence
           will switch S-nail to a reproducible mode (https://reproducible-builds.org) which uses
           deterministic random numbers, a special fixated pseudo LOGNAME and more.  This
           operation mode is used for development and by software packagers.  [v15 behaviour may
           differ] Currently an invalid setting is only ignored, rather than causing a program
           abortion.

                 $ SOURCE_DATE_EPOCH=`date +%s` s-nail

     TERM  [Option] The terminal type for which output is to be prepared.  For extended colour
           and font control please refer to Coloured display, and for terminal management in
           general to On terminal control and line editor.

     TMPDIR
           Except for the root user this variable defines the directory for temporary files to be
           used instead of /tmp (or the given compile-time constant) if set, existent, accessible
           as well as read- and writable.  This variable is only used when it resides in the
           process environment, but S-nail will ensure at startup that this environment variable
           is updated to contain a usable temporary directory.

     USER  Identical to LOGNAME (see there), but this variable is not standardized, should
           therefore not be used, and is only corrected if already set.

     VISUAL
           Pathname of the text editor to use for the visual command and ~v (see COMMAND
           ESCAPES); EDITOR is used for a less display oriented editor.

FILES

     ~/.mailrc
           User-specific file giving initial commands, one of the Resource files.  The actual
           value is read from MAILRC.

     /etc/s-nail.rc
           System wide initialization file, one of the Resource files.  The actual value is read
           from system-mailrc.

     ~/.mailcap
           [Option] Personal MIME type handler definition file, see The Mailcap files.  This
           location is part of the RFC 1524 standard search path, which is a configuration option
           and can be overridden via MAILCAPS.

     /etc/mailcap
           [Option] System wide MIME type handler definition file, see The Mailcap files.  This
           location is part of the RFC 1524 standard search path, which is a configuration option
           and can be overridden via

     ~/mbox
           The default value for MBOX.

     ~/.mime.types
           Personal MIME types, see The mime.types files.

     /etc/mime.types
           System wide MIME types, see The mime.types files.

     ~/.netrc
           [v15-compat][Option] The default location of the user's .netrc file – the section The
           .netrc file documents the file format.  The actually used path can be overridden via
           NETRC.

     /dev/null
           The data sink null(4).

   Resource files
     Upon startup S-nail reads in several resource files, in order:

     /etc/s-nail.rc
           System wide initialization file (system-mailrc).  Reading of this file can be
           suppressed, either by using the -: (and according argument) or -n command line
           options, or by setting the ENVIRONMENT variable MAILX_NO_SYSTEM_RC.

     ~/.mailrc
           File giving initial commands.  A different file can be chosen by setting the
           ENVIRONMENT variable MAILRC.  Reading of this file can be suppressed with the -:
           command line option.

     mailx-extra-rc
           Defines a startup file to be read after all other resource files.  It can be used to
           specify settings that are not understood by other mailx(1) implementations, for
           example.  This variable is only honoured when defined in a resource file, e.g., it is
           one of the INTERNAL VARIABLES.

     The content of these files is interpreted as follows:

     ·   The whitespace characters space, tabulator and newline, as well as those defined by the
         variable ifs, are removed from the beginning and end of input lines.
     ·   Empty lines are ignored.
     ·   Any other line is interpreted as a command.  It may be spread over multiple input lines
         if the newline character is “escaped” by placing a reverse solidus character ‘\’ as the
         last character of the line; whereas any leading whitespace of follow lines is ignored,
         trailing whitespace before a escaped newline remains in the input.
     ·   If the line (content) starts with the number sign ‘#’ then it is a comment-command and
         also ignored.  (The comment-command is a real command, which does nothing, and therefore
         the usual follow lines mechanism applies!)

     Unless S-nail is about to enter interactive mode syntax errors that occur while loading
     these files are treated as errors and cause program exit.  More files with syntactically
     equal content can be sourceed.  The following, saved in a file, would be an examplary
     content:

            # This line is a comment command.  And y\
               es, it is really continued here.
           set debug \
               verbose
               set editheaders

   The mime.types files
     As stated in HTML mail and MIME attachments S-nail needs to learn about MIME (Multipurpose
     Internet Mail Extensions) media types in order to classify message and attachment content.
     One source for them are mime.types files, the loading of which can be controlled by setting
     the variable mimetypes-load-control.  Another is the command mimetype, which also offers
     access to S-nails MIME type cache.  mime.types files have the following syntax:

           type/subtype extension [extension ...]
           # E.g., text/html html htm

     where ‘type/subtype’ define the MIME media type, as standardized in RFC 2046: ‘type’ is used
     to declare the general type of data, while the ‘subtype’ specifies a specific format for
     that type of data.  One or multiple filename ‘extension’s, separated by whitespace, can be
     bound to the media type format.  Comments may be introduced anywhere on a line with a number
     sign ‘#’, causing the remaining line to be discarded.  S-nail also supports an extended,
     non-portable syntax in especially crafted files, which can be loaded via the alternative
     value syntax of mimetypes-load-control, and prepends an optional ‘type-marker’:

           [type-marker ]type/subtype extension [extension ...]

     The following type markers are supported:

     @     Treat message parts with this content as plain text.
     @t    The same as plain @.
     @h    Treat message parts with this content as HTML tagsoup.  If the [Option]al HTML-
           tagsoup-to-text converter is not available treat the content as plain text instead.
     @H    Likewise @h, but instead of falling back to plain text require an explicit content
           handler to be defined.
     @q    If no handler can be found a text message is displayed which says so.  This can be
           annoying, for example signatures serve a contextual purpose, their content is of no
           use by itself.  This marker will avoid displaying the text message.

     Further reading: for sending messages: mimetype, mime-allow-text-controls,
     mimetypes-load-control.  For reading etc. messages: HTML mail and MIME attachments, The
     Mailcap files, mimetype, mime-counter-evidence, mimetypes-load-control, pipe-TYPE/SUBTYPE,
     pipe-EXTENSION.

   The Mailcap files
     This feature is not available in v14.9.0, sorry! RFC 1524 defines a “User Agent
     Configuration Mechanism” which S-nail [Option]ally supports (see HTML mail and MIME
     attachments).  It defines a file format to be used to inform mail user agent programs about
     the locally-installed facilities for handling various data formats, i.e., about commands and
     how they can be used to display, edit et cetera MIME part contents, as well as a default
     path search that includes multiple possible locations of “mailcap” files and the MAILCAPS
     environment variable that can be used to overwrite that (repeating here that it is not a
     search path, but instead a path search specification).  Any existing files will be loaded in
     sequence, appending any content to the list of MIME type handler directives.

     “Mailcap” files consist of a set of newline separated entries.  Comment lines start with a
     number sign ‘#’ (in the first column!) and are ignored.  Empty lines are also ignored.  All
     other lines form individual entries that must adhere to the syntax described below.  To
     extend a single entry (not comment) its line can be continued on follow lines if newline
     characters are “escaped” by preceding them with the reverse solidus character ‘\’.  The
     standard does not specify how leading whitespace of follow lines is to be treated, therefore
     S-nail retains it.

     “Mailcap” entries consist of a number of semicolon ‘;’ separated fields, and the reverse
     solidus ‘\’ character can be used to escape any following character including semicolon and
     itself.  The first two fields are mandatory and must occur in the specified order, the
     remaining fields are optional and may appear in any order.  Leading and trailing whitespace
     of content is ignored (removed).

     The first field defines the MIME ‘TYPE/SUBTYPE’ the entry is about to handle (case-
     insensitively, and no reverse solidus escaping is possible in this field).  If the subtype
     is specified as an asterisk ‘*’ the entry is meant to match all subtypes of the named type,
     e.g., ‘audio/*’ would match any audio type.  The second field defines the shell command
     which shall be used to “display” MIME parts of the given type; it is implicitly called the
     view command.

     For data “consuming” shell commands message (MIME part) data is passed via standard input
     unless the given shell command includes one or more instances of the (unquoted) string ‘%s’,
     in which case these instances will be replaced with a temporary filename and the data will
     have been stored in the file that is being pointed to.  Likewise, for data “producing” shell
     commands data is assumed to be generated on standard output unless the given command
     includes (one ore multiple) ‘%s’.  In any case any given ‘%s’ format is replaced with a(n
     already) properly quoted filename.  Note that when a command makes use of a temporary file
     via ‘%s’ then S-nail will remove it again, as if the x-mailx-tmpfile, x-mailx-tmpfile-fill
     and x-mailx-tmpfile-unlink flags had been set; see below for more.

     The optional fields either define a shell command or an attribute (flag) value, the latter
     being a single word and the former being a keyword naming the field followed by an equals
     sign ‘=’ succeeded by a shell command, and as usual for any “Mailcap” content any whitespace
     surrounding the equals sign will be removed, too.  Optional fields include the following:

     compose
           A program that can be used to compose a new body or body part in the given format.
           (Currently unused.)

     composetyped
           Similar to the compose field, but is to be used when the composing program needs to
           specify the ‘Content-type:’ header field to be applied to the composed data.
           (Currently unused.)

     edit  A program that can be used to edit a body or body part in the given format.
           (Currently unused.)

     print
           A program that can be used to print a message or body part in the given format.
           (Currently unused.)

     test  Specifies a program to be run to test some condition, e.g., the machine architecture,
           or the window system in use, to determine whether or not this mailcap entry applies.
           If the test fails, a subsequent mailcap entry should be sought; also see
           x-mailx-test-once.

     needsterminal
           This flag field indicates that the given shell command must be run on an interactive
           terminal.  S-nail will temporarily release the terminal to the given command in
           interactive mode, in non-interactive mode this entry will be entirely ignored; this
           flag implies x-mailx-noquote.

     copiousoutput
           A flag field which indicates that the output of the view command will be an extended
           stream of textual output that can be (re)integrated into S-nail's normal visual
           display.  It is mutually exclusive with needsterminal.

     textualnewlines
           A flag field which indicates that this type of data is line-oriented and that, if
           encoded in ‘base64’, all newlines should be converted to canonical form (CRLF) before
           encoding, and will be in that form after decoding.  (Currently unused.)

     nametemplate
           This field gives a filename format, in which ‘%s’ will be replaced by a random string,
           the joined combination of which will be used as the filename denoted by
           MAILX_FILENAME_TEMPORARY.  One could specify that a GIF file being passed to an image
           viewer should have a name ending in ‘.gif’ by using ‘nametemplate=%s.gif’.  Note that
           S-nail ignores the name template unless that solely specifies a filename suffix that
           consists of (ASCII) alphabetic and numeric characters, the underscore and dot only.

     x11-bitmap
           Names a file, in X11 bitmap (xbm) format, which points to an appropriate icon to be
           used to visually denote the presence of this kind of data.  This field is not used by
           S-nail.

     description
           A textual description that describes this type of data.

     x-mailx-even-if-not-interactive
           An extension flag test field — by default handlers without copiousoutput are entirely
           ignored in non-interactive mode, but if this flag is set then their use will be
           considered.  It is an error if this flag is set for commands that use the flag
           needsterminal.

     x-mailx-noquote
           An extension flag field that indicates that even a copiousoutput view command shall
           not be used to generate message quotes (as it would be by default).

     x-mailx-async
           Extension flag field that denotes that the given view command shall be executed
           asynchronously, without blocking S-nail.  Cannot be used in conjunction with
           needsterminal.

     x-mailx-test-once
           Extension flag which denotes whether the given test command shall be evaluated once
           only and the (boolean) result be cached.  This is handy if some global unchanging
           condition is to be queried, like “running under the X Window System”.

     x-mailx-tmpfile
           Extension flag field that requests creation of a zero-sized temporary file, the name
           of which is to be placed in the environment variable MAILX_FILENAME_TEMPORARY.  It is
           an error to use this flag with commands that include a ‘%s’ format.

     x-mailx-tmpfile-fill
           Normally the MIME part content is passed to the handler via standard input; if this
           flag is set then the data will instead be written into the implied x-mailx-tmpfile.
           In order to cause deletion of the temporary file you will have to set
           x-mailx-tmpfile-unlink explicitly!  It is an error to use this flag with commands that
           include a ‘%s’ format.

     x-mailx-tmpfile-unlink
           Extension flag field that requests that the temporary file shall be deleted
           automatically when the command loop is entered again at latest.  (Do not use this for
           asynchronous handlers.)  It is an error to use this flag with commands that include a
           ‘%s’ format, or in conjunction with x-mailx-async, or without also setting
           x-mailx-tmpfile or x-mailx-tmpfile-fill.

     x-mailx-tmpfile-keep
           Using the string ‘%s’ implies the three tmpfile related flags above, but if you want,
           e.g., x-mailx-async and deal with the temporary file yourself, you can add in this
           flag to forcefully ignore x-mailx-tmpfile-unlink.

     The standard includes the possibility to define any number of additional entry fields,
     prefixed by ‘x-’.  Flag fields apply to the entire “Mailcap” entry — in some unusual cases,
     this may not be desirable, but differentiation can be accomplished via separate entries,
     taking advantage of the fact that subsequent entries are searched if an earlier one does not
     provide enough information.  E.g., if a view command needs to specify the needsterminal
     flag, but the compose command shall not, the following will help out the latter (with
     enabled debug or an increased verbose level S-nail will show information about handler
     evaluation):

           application/postscript; ps-to-terminal %s; needsterminal
           application/postscript; ps-to-terminal %s; compose=idraw %s

     In fields any occurrence of the format string ‘%t’ will be replaced by the ‘TYPE/SUBTYPE’
     specification.  Named parameters from the ‘Content-type:’ field may be placed in the command
     execution line using ‘%{’ followed by the parameter name and a closing ‘}’ character.  The
     entire parameter should appear as a single command line argument, regardless of embedded
     spaces; thus:

           # Message
           Content-type:  multipart/mixed; boundary=42

           # Mailcap file
           multipart/*; /usr/local/bin/showmulti \
             %t %{boundary}  ;  composetyped  = /usr/local/bin/makemulti

           # Executed shell command
           /usr/local/bin/showmulti multipart/mixed 42

     Note that S-nail does not support handlers for multipart MIME parts as shown in this example
     (as of today).  S-nail does not support the additional formats ‘%n’ and ‘%F’.  An example
     file, also showing how to properly deal with the expansion of ‘%s’, which includes any
     quotes that are necessary to make it a valid shell argument by itself and thus will cause
     undesired behaviour when placed in additional user-provided quotes:

           # Comment line
           text/richtext; richtext %s; copiousoutput

           text/x-perl; perl -cWT %s

           application/pdf; \
             infile=%s\; \
               trap "rm -f ${infile}" EXIT\; \
               trap "exit 75" INT QUIT TERM\; \
               mupdf %s; \
             x-mailx-async; x-mailx-tmpfile-keep

           application/*; echo "This is \"%t\" but \
               is 50 \% Greek to me" \; < %s head -c 1024 | cat -vet; \
             copiousoutput; x-mailx-noquote

     Further reading: HTML mail and MIME attachments, The mime.types files, mimetype, MAILCAPS,
     mime-counter-evidence, pipe-TYPE/SUBTYPE, pipe-EXTENSION.

   The .netrc file
     The .netrc file contains user credentials for machine accounts.  The default location ~/
     .netrc may be overridden by the NETRC environment variable.  It is possible to load
     encrypted .netrc files by using an appropriate value in netrc-pipe.

     The file consists of space, tabulator or newline separated tokens.  S-nail implements a
     parser that supports a superset of the original BSD syntax, but users should nonetheless be
     aware of portability glitches of that file format, shall their .netrc be usable across
     multiple programs and platforms:

     ·   BSD does not support single, but only double quotation marks, e.g., ‘password="pass with
         spaces"’.
     ·   BSD (only?) supports escaping of single characters via a reverse solidus (e.g., a space
         can be escaped via ‘\ ’), in- as well as outside of a quoted string.
     ·   BSD does not require a final quotation mark of the last user input token.
     ·   The original BSD (Berknet) parser also supported a format which allowed tokens to be
         separated with commas – whereas at least Hewlett-Packard still seems to support this
         syntax, S-nail does not!
     ·   As a non-portable extension some widely-used programs support shell-style comments: if
         an input line starts, after any amount of whitespace, with a number sign ‘#’, then the
         rest of the line is ignored.
     ·   Whereas other programs may require that the .netrc file is accessible by only the user
         if it contains a password token for any other login than “anonymous”, S-nail will always
         require these strict permissions.

     Of the following list of supported tokens S-nail only uses (and caches) machine, login and
     password.  At runtime the command netrc can be used to control S-nail's .netrc cache.

     machine name
           The hostname of the entries' machine, lowercase-normalized by S-nail before use.  Any
           further file content, until either end-of-file or the occurrence of another machine or
           a default first-class token is bound (only related) to the machine name.

           As an extension that should not be the cause of any worries S-nail supports a single
           wildcard prefix for name:

                 machine *.example.com login USER password PASS
                 machine pop3.example.com login USER password PASS
                 machine smtp.example.com login USER password PASS

           which would match ‘xy.example.com’ as well as ‘pop3.example.com’, but neither
           ‘example.com’ nor ‘local.smtp.example.com’.  Note that in the example neither
           ‘pop3.example.com’ nor ‘smtp.example.com’ will be matched by the wildcard, since the
           exact matches take precedence (it is however faster to specify it the other way
           around).

     default
           This is the same as machine except that it is a fallback entry that is used shall none
           of the specified machines match; only one default token may be specified, and it must
           be the last first-class token.

     login name
           The user name on the remote machine.

     password string
           The user's password on the remote machine.

     account string
           Supply an additional account password.  This is merely for FTP purposes.

     macdef name
           Define a macro.  A macro is defined with the specified name; it is formed from all
           lines beginning with the next line and continuing until a blank line is (consecutive
           newline characters are) encountered.  (Note that macdef entries cannot be utilized by
           multiple machines, too, but must be defined following the machine they are intended to
           be used with.)  If a macro named init exists, it is automatically run as the last step
           of the login process.  This is merely for FTP purposes.

EXAMPLES

   An example configuration
           # This example assumes v15.0 compatibility mode
           set v15-compat

           # Request strict TLL transport layer security checks
           set tls-verify=strict

           # Where are the up-to-date TLS certificates?
           # (Since we manage up-to-date ones explicitly, do not use any,
           # possibly outdated, default certificates shipped with OpenSSL)
           #set tls-ca-dir=/etc/ssl/certs
           set tls-ca-file=/etc/ssl/certs/ca-certificates.crt
           set tls-ca-no-defaults
           #set tls-ca-flags=partial-chain
           wysh set smime-ca-file="${tls-ca-file}" \
             smime-ca-no-defaults #smime-ca-flags="${tls-ca-flags}"

           # This could be outsourced to a central configuration file via
           # tls-config-file plus tls-config-module if the used library allows.
           # CipherString: explicitly define the list of ciphers, which may
           #   improve security, especially with protocols older than TLS v1.2.
           #   See ciphers(1).  Possibly best to only use tls-config-pairs-HOST
           #   (or -USER@HOST), as necessary, again..
           #   Note that TLSv1.3 uses Ciphersuites= instead, which will join
           #   with CipherString (if protocols older than v1.3 are allowed)
           # Curves: especially with TLSv1.3 curves selection may be desired.
           # MinProtocol,MaxProtocol: do not use protocols older than TLS v1.2.
           #   Change this only when the remote server does not support it:
           #   maybe use chain support via tls-config-pairs-HOST / -USER@HOST
           #   to define such explicit exceptions, then, e.g.
           #     MinProtocol=TLSv1.1
           if [ "$tls-features" =% +ctx-set-maxmin-proto ]
             wysh set tls-config-pairs='\
                 CipherString=TLSv1.2:!aNULL:!eNULL:@STRENGTH,\
                 Curves=P-521:P-384:P-256,\
                 MinProtocol=TLSv1.1'
           else
             wysh set tls-config-pairs='\
                 CipherString=TLSv1.2:!aNULL:!eNULL:@STRENGTH,\
                 Curves=P-521:P-384:P-256,\
                 Protocol=-ALL\,+TLSv1.1 \, +TLSv1.2\, +TLSv1.3'
           endif

           # Essential setting: select allowed character sets
           set sendcharsets=utf-8,iso-8859-1

           # A very kind option: when replying to a message, first try to
           # use the same encoding that the original poster used herself!
           set reply-in-same-charset

           # When replying, do not merge From: and To: of the original message
           # into To:.  Instead old From: -> new To:, old To: -> merge Cc:.
           set recipients-in-cc

           # When sending messages, wait until the Mail-Transfer-Agent finishs.
           # Only like this you will be able to see errors reported through the
           # exit status of the MTA (including the built-in SMTP one)!
           set sendwait

           # Only use built-in MIME types, no mime.types(5) files
           set mimetypes-load-control

           # Default directory where we act in (relative to $HOME)
           set folder=mail
           # A leading "+" (often) means: under *folder*
           # *record* is used to save copies of sent messages
           set MBOX=+mbox.mbox DEAD=+dead.txt \
             record=+sent.mbox record-files record-resent

           # Make "file mymbox" and "file myrec" go to..
           shortcut mymbox %:+mbox.mbox myrec +sent.mbox

           # Not really optional, e.g., for S/MIME
           set from='Your Name <address@exam.ple>'

           # It may be necessary to set hostname and/or smtp-hostname
           # if the "SERVER" of mta and "domain" of from do not match.
           # The `urlencode' command can be used to encode USER and PASS
           set mta=(smtps?|submissions?)://[USER[:PASS]@]SERVER[:PORT] \
             smtp-auth=login/plain... \
             smtp-use-starttls

           # Never refuse to start into interactive mode, and more
           set emptystart \
             colour-pager crt= \
             followup-to followup-to-honour=ask-yes fullnames \
             history-file=+.s-nailhist history-size=-1 history-gabby \
             mime-counter-evidence=0b1111 \
             prompt='?\$?!\$!/\$^ERRNAME[\$account#\$mailbox-display]? ' \
             reply-to-honour=ask-yes \
             umask=

           # Only include the selected header fields when typing messages
           headerpick type retain from_ date from to cc subject \
             message-id mail-followup-to reply-to
           # ...when forwarding messages
           headerpick forward retain subject date from to cc
           # ...when saving message, etc.
           #headerpick save ignore ^Original-.*$ ^X-.*$

           # Some mailing lists
           mlist '@xyz-editor\.xyz$' '@xyzf\.xyz$'
           mlsubscribe '^xfans@xfans\.xyz$'

           # Handle a few file extensions (to store MBOX databases)
           filetype bz2 'bzip2 -dc' 'bzip2 -zc' \
             gz 'gzip -dc' 'gzip -c'  xz 'xz -dc' 'xz -zc' \
             zst 'zstd -dc' 'zstd -19 -zc' \
             zst.pgp 'gpg -d | zstd -dc' 'zstd -19 -zc | gpg -e'

           # A real life example of a very huge free mail provider
           # Instead of directly placing content inside `account',
           # we `define' a macro: like that we can switch "accounts"
           # from within *on-compose-splice*, for example!
           define XooglX {
             set folder=~/spool/XooglX inbox=+syste.mbox sent=+sent
             set from='Your Name <address@examp.ple>'

             set pop3-no-apop-pop.gmXil.com
             shortcut pop %:pop3s://pop.gmXil.com
             shortcut imap %:imaps://imap.gmXil.com
             # Or, entirely IMAP based setup
             #set folder=imaps://imap.gmail.com record="+[Gmail]/Sent Mail" \
             #   imap-cache=~/spool/cache

             set mta=smtp://USER:PASS@smtp.gmXil.com smtp-use-starttls
             # Alternatively:
             set mta=smtps://USER:PASS@smtp.gmail.com:465
           }
           account XooglX {
             \call XooglX
           }

           # Here is a pretty large one which does not allow sending mails
           # if there is a domain name mismatch on the SMTP protocol level,
           # which would bite us if the value of from does not match, e.g.,
           # for people who have a sXXXXeforge project and want to speak
           # with the mailing list under their project account (in from),
           # still sending the message through their normal mail provider
           define XandeX {
             set folder=~/spool/XandeX inbox=+syste.mbox sent=+sent
             set from='Your Name <address@exam.ple>'

             shortcut pop %:pop3s://pop.yaXXex.com
             shortcut imap %:imaps://imap.yaXXex.com

             set mta=smtps://USER:PASS@smtp.yaXXex.com:465 \
               hostname=yaXXex.com smtp-hostname=
           }
           account XandeX {
             \call Xandex
           }

           # Create some new commands so that, e.g., `ls /tmp' will..
           commandalias lls '!ls ${LS_COLOUR_FLAG} -aFlrS'
           commandalias llS '!ls ${LS_COLOUR_FLAG} -aFlS'

           set pipe-message/external-body='@* echo $MAILX_EXTERNAL_BODY_URL'

           # We do not support gpg(1) directly yet.  But simple --clearsign'd
           # message parts can be dealt with as follows:
           define V {
             localopts yes
             wysh set pipe-text/plain=$'@*#++=@\
               < "${MAILX_FILENAME_TEMPORARY}" awk \
                   -v TMPFILE="${MAILX_FILENAME_TEMPORARY}" \'\
                 BEGIN{done=0}\
                 /^-----BEGIN PGP SIGNED MESSAGE-----/,/^$/ {\
                   if(done++ != 0)\
                     next;\
                   print "--- GPG --verify ---";\
                   system("gpg --verify " TMPFILE " 2>&1");\
                   print "--- GPG --verify ---";\
                   print "";\
                   next;\
                 }\
                 /^-----BEGIN PGP SIGNATURE-----/,\
                     /^-----END PGP SIGNATURE-----/{\
                   next;\
                 }\
                 {print}\
               \''
               print
           }
           commandalias V '\'call V

     When storing passwords in ~/.mailrc appropriate permissions should be set on this file with
     ‘$ chmod 0600 ~/.mailrc’.  If the [Option]al netrc-lookup is available user credentials can
     be stored in the central ~/.netrc file instead; e.g., here is a different version of the
     example account that sets up SMTP and POP3:

           define XandeX {
             set folder=~/spool/XandeX inbox=+syste.mbox sent=+sent
             set from='Your Name <address@exam.ple>'
             set netrc-lookup
             # Load an encrypted ~/.netrc by uncommenting the next line
             #set netrc-pipe='gpg -qd ~/.netrc.pgp'

             set mta=smtps://smtp.yXXXXx.ru:465 \
                 smtp-hostname= hostname=yXXXXx.com
             set pop3-keepalive=240 pop3-no-apop-pop.yXXXXx.ru
             commandalias xp fi pop3s://pop.yXXXXx.ru
           }
           account XandeX {
             \call XandeX
           }

     and, in the ~/.netrc file:

           machine *.yXXXXx.ru login USER password PASS

     This configuration should now work just fine:

           $ echo text | s-nail -dvv -AXandeX -s Subject user@exam.ple

   S/MIME step by step
     [Option] The first thing that is needed for Signed and encrypted messages with S/MIME is a
     personal certificate, and a private key.  The certificate contains public information, in
     particular a name and email address(es), and the public key that can be used by others to
     encrypt messages for the certificate holder (the owner of the private key), and to verify
     signed messages generated with that certificate('s private key).  Whereas the certificate is
     included in each signed message, the private key must be kept secret.  It is used to decrypt
     messages that were previously encrypted with the public key, and to sign messages.

     For personal use it is recommended that get a S/MIME certificate from one of the major CAs
     on the Internet.  Many CAs offer such certificates for free.  Usually offered is a combined
     certificate and private key in PKCS#12 format which S-nail does not accept directly.  To
     convert it to PEM format, the following shell command can be used; please read on for how to
     use these PEM files.

           $ openssl pkcs12 -in cert.p12 -out certpem.pem -clcerts -nodes
           $ # Alternatively
           $ openssl pkcs12 -in cert.p12 -out cert.pem -clcerts -nokeys
           $ openssl pkcs12 -in cert.p12 -out key.pem -nocerts -nodes

     There is also https://www.CAcert.org which issues client and server certificates to members
     of their community for free; their root certificate (https://www.cacert.org/certs/root.crt)
     is often not in the default set of trusted CA root certificates, though, which means their
     root certificate has to be downloaded separately, and needs to be part of the S/MIME
     certificate validation chain by including it in smime-ca-dir or as a vivid member of the
     smime-ca-file.  But let us take a step-by-step tour on how to setup S/MIME with a
     certificate from CAcert.org despite this situation!

     First of all you will have to become a member of the CAcert.org community, simply by
     registrating yourself via the web interface.  Once you are, create and verify all email
     addresses you want to be able to create signed and encrypted messages for/with using the
     corresponding entries of the web interface.  Now ready to create S/MIME certificates, so let
     us create a new “client certificate”, ensure to include all email addresses that should be
     covered by the certificate in the following web form, and also to use your name as the
     “common name”.

     Create a private key and a certificate request on your local computer (please see the manual
     pages of the used commands for more in-depth knowledge on what the used arguments etc. do):

           $ openssl req -nodes -newkey rsa:4096 -keyout key.pem -out creq.pem

     Afterwards copy-and-paste the content of “creq.pem” into the certificate-request (CSR) field
     of the web form on the CAcert.org website (you may need to unfold some “advanced options” to
     see the corresponding text field).  This last step will ensure that your private key (which
     never left your box) and the certificate belong together (through the public key that will
     find its way into the certificate via the certificate-request).  You are now ready and can
     create your CAcert certified certificate.  Download and store or copy-and-paste it as
     “pub.crt”.

     Yay.  In order to use your new S/MIME setup a combined private key/public key (certificate)
     file has to be created:

           $ cat key.pem pub.crt > ME@HERE.com.paired

     This is the file S-nail will work with.  If you have created your private key with a
     passphrase then S-nail will ask you for it whenever a message is signed or decrypted, unless
     this operation has been automated as described in Signed and encrypted messages with S/MIME.
     Set the following variables to henceforth use S/MIME (setting smime-ca-file is of interest
     for verification only):

           ? set smime-ca-file=ALL-TRUSTED-ROOT-CERTS-HERE \
               smime-sign-cert=ME@HERE.com.paired \
               smime-sign-digest=SHA512 \
               smime-sign

   Using CRLs with S/MIME or TLS
     [Option] Certification authorities (CAs) issue certificate revocation lists (CRLs) on a
     regular basis.  These lists contain the serial numbers of certificates that have been
     declared invalid after they have been issued.  Such usually happens because the private key
     for the certificate has been compromised, because the owner of the certificate has left the
     organization that is mentioned in the certificate, etc.  To seriously use S/MIME or TLS
     verification, an up-to-date CRL is required for each trusted CA.  There is otherwise no
     method to distinguish between valid and invalidated certificates.  S-nail currently offers
     no mechanism to fetch CRLs, nor to access them on the Internet, so they have to be retrieved
     by some external mechanism.

     S-nail accepts CRLs in PEM format only; CRLs in DER format must be converted, like, e.g.:

           $ openssl crl -inform DER -in crl.der -out crl.pem

     To tell S-nail about the CRLs, a directory that contains all CRL files (and no other files)
     must be created.  The smime-crl-dir or tls-crl-dir variables, respectively, must then be set
     to point to that directory.  After that, S-nail requires a CRL to be present for each CA
     that is used to verify a certificate.

FAQ

     In general it is a good idea to turn on debug (-d) and / or verbose (-v, twice) if something
     does not work well.  Very often a diagnostic message can be produced that leads to the
     problems' solution.

   S-nail shortly hangs on startup
     This can have two reasons, one is the necessity to wait for a file lock and cannot be
     helped, the other being that S-nail calls the function uname(2) in order to query the
     nodename of the box (sometimes the real one is needed instead of the one represented by the
     internal variable hostname).  One may have varying success by ensuring that the real
     hostname and ‘localhost’ have entries in /etc/hosts, or, more generally, that the name
     service is properly setup – and does hostname(1) return the expected value?  Does this local
     hostname have a domain suffix?  RFC 6762 standardized the link-local top-level domain
     ‘.local’, try again after adding an (additional) entry with this extension.

   I cannot login to Google mail aka GMail
     Since 2014 some free service providers classify programs as “less secure” unless they use a
     special authentication method (OAuth 2.0) which was not standardized for non-HTTP protocol
     authentication token query until August 2015 (RFC 7628).

     Different to Kerberos / GSSAPI, which is developed since the mid of the 1980s, where a user
     can easily create a local authentication ticket for her- and himself with the locally
     installed kinit(1) program, that protocol has no such local part but instead requires a
     world-wide-web query to create or fetch a token; since there is no local cache this query
     would have to be performed whenever S-nail is invoked (in interactive sessions situation may
     differ).

     S-nail does not support OAuth.  Because of this it is necessary to declare S-nail a “less
     secure app” (on the providers account web page) in order to read and send mail.  However, it
     also seems possible to take the following steps instead:

     1.   give the provider the number of a mobile phone,
     2.   enable “2-Step Verification”,
     3.   create an application specific password (16 characters), and
     4.   use that special password instead of the real Google account password in S-nail (for
          more on that see the section On URL syntax and credential lookup).

   Not "defunctional", but the editor key does not work
     It can happen that the terminal library (see On terminal control and line editor, bind,
     termcap) reports different codes than the terminal really sends, in which case S-nail will
     tell that a key binding is functional, but will not be able to recognize it because the
     received data does not match anything expected.  Especially without the [Option]al terminal
     capability library support one reason for this may be that the (possibly even non-existing)
     keypad is not turned on and the resulting layout reports the keypad control codes for the
     normal keyboard keys.  The verbose listing of bindings will show the byte sequences that are
     expected.

     To overcome the situation, use, e.g., the program cat(1), in conjunction with the command
     line option -v, if available, to see the byte sequences which are actually produced by
     keypresses, and use the variable termcap to make S-nail aware of them.  E.g., the terminal
     this is typed on produces some false sequences, here an example showing the shifted home
     key:

           ? set verbose
           ? bind*
           # 1B 5B=[ 31=1 3B=; 32=2 48=H
             bind base :kHOM z0
           ? x
           $ cat -v
           ^[[H
           $ s-nail -v -Stermcap='kHOM=\E[H'
           ? bind*
           # 1B 5B=[ 48=H
             bind base :kHOM z0

   Can S-nail git-send-email?
     Yes.  Put (at least parts of) the following in your ~/.gitconfig:

           [sendemail]
           smtpserver = /usr/bin/s-mailx
           smtpserveroption = -t
           #smtpserveroption = -Sexpandaddr
           smtpserveroption = -Athe-account-you-need
           ##
           suppresscc = all
           suppressfrom = false
           assume8bitEncoding = UTF-8
           #to = /tmp/OUT
           confirm = always
           chainreplyto = true
           multiedit = false
           thread = true
           quiet = true
           annotate = true

IMAP CLIENT

     [Option]ally there is IMAP client support available.  This part of the program is obsolete
     and will vanish in v15 with the large MIME and I/O layer rewrite, because it uses old-style
     blocking I/O and makes excessive use of signal based long code jumps.  Support can hopefully
     be readded later based on a new-style I/O, with SysV signal handling.  In fact the IMAP
     support had already been removed from the codebase, but was reinstantiated on user demand:
     in effect the IMAP code is at the level of S-nail v14.8.16 (with imapcodec being the sole
     exception), and should be treated with some care.

     IMAP uses the ‘imap://’ and ‘imaps://’ protocol prefixes, and an IMAP-based folder may be
     used.  IMAP URLs (paths) undergo inspections and possible transformations before use (and
     the command imapcodec can be used to manually apply them to any given argument).  Hierarchy
     delimiters are normalized, a step which is configurable via the imap-delim variable chain,
     but defaults to the first seen delimiter otherwise.  S-nail supports internationalised IMAP
     names, and en- and decodes the names from and to the ttycharset as necessary and possible.
     If a mailbox name is expanded (see Filename transformations) to an IMAP mailbox, all names
     that begin with `+' then refer to IMAP mailboxes below the folder target box, while folder
     names prefixed by `@' refer to folders below the hierarchy base, e.g., the following lists
     all folders below the current one when in an IMAP mailbox: ‘folders @’.

     Note: some IMAP servers do not accept the creation of mailboxes in the hierarchy base, but
     require that they are created as subfolders of `INBOX' – with such servers a folder name of
     the form

           imaps://mylogin@imap.myisp.example/INBOX.

     should be used (the last character is the server's hierarchy delimiter).  The following
     IMAP-specific commands exist:

     cache
           Only applicable to cached IMAP mailboxes; takes a message list and reads the specified
           messages into the IMAP cache.

     connect
           If operating in disconnected mode on an IMAP mailbox, switch to online mode and
           connect to the mail server while retaining the mailbox status.  See the description of
           the disconnected variable for more information.

     disconnect
           If operating in online mode on an IMAP mailbox, switch to disconnected mode while
           retaining the mailbox status.  See the description of the disconnected variable for
           more.  A list of messages may optionally be given as argument; the respective messages
           are then read into the cache before the connection is closed, thus ‘disco *’ makes the
           entire mailbox available for disconnected use.

     imap  Sends command strings directly to the current IMAP server.  S-nail operates always in
           IMAP `selected state' on the current mailbox; commands that change this will produce
           undesirable results and should be avoided.  Useful IMAP commands are:

                 create         Takes the name of an IMAP mailbox as an argument and creates it.

                 getquotaroot   (RFC 2087) Takes the name of an IMAP mailbox as an argument and
                                prints the quotas that apply to the mailbox.  Not all IMAP
                                servers support this command.

                 namespace      (RFC 2342) Takes no arguments and prints the Personal Namespaces,
                                the Other User's Namespaces and the Shared Namespaces.  Each
                                namespace type is printed in parentheses; if there are multiple
                                namespaces of the same type, inner parentheses separate them.
                                For each namespace a prefix and a hierarchy separator is listed.
                                Not all IMAP servers support this command.

     imapcodec
           Perform IMAP path transformations.  Supports vput (see Command modifiers), and manages
           the error number !.  The first argument specifies the operation: e[ncode] normalizes
           hierarchy delimiters (see imap-delim) and converts the strings from the locale
           ttycharset to the internationalized variant used by IMAP, d[ecode] performs the
           reverse operation.

     The following IMAP-specific internal variables exist:

     disconnected
           (Boolean) When an IMAP mailbox is selected and this variable is set, no connection to
           the server is initiated.  Instead, data is obtained from the local cache (see
           imap-cache).  Mailboxes that are not present in the cache and messages that have not
           yet entirely been fetched from the server are not available; to fetch all messages in
           a mailbox at once, the command `copy * /dev/null' can be used while still in connected
           mode.  Changes that are made to IMAP mailboxes in disconnected mode are queued and
           committed later when a connection to that server is made.  This procedure is not
           completely reliable since it cannot be guaranteed that the IMAP unique identifiers
           (UIDs) on the server still match the ones in the cache at that time.  Data is saved to
           DEAD when this problem occurs.

     disconnected-USER@HOST
           The specified account is handled as described for the disconnected variable above, but
           other accounts are not affected.

     imap-auth-USER@HOST, imap-auth
           Sets the IMAP authentication method.  Valid values are `login' for the usual password-
           based authentication (the default), `cram-md5', which is a password-based
           authentication that does not send the password over the network in clear text, and
           `gssapi' for GSS-API based authentication.

     imap-cache
           Enables caching of IMAP mailboxes.  The value of this variable must point to a
           directory that is either existent or can be created by S-nail.  All contents of the
           cache can be deleted by S-nail at any time; it is not safe to make assumptions about
           them.

     imap-delim-USER@HOST, imap-delim-HOST, imap-delim
           The hierarchy separator used by the IMAP server.  Whenever an IMAP path is specified
           it will undergo normalization.  One of the normalization steps is the squeezing and
           adjustment of hierarchy separators.  If this variable is set, any occurrence of any
           character of the given value that exists in the path will be replaced by the first
           member of the value; an empty value will cause the default to be used, it is ‘/.’.  If
           not set, we will reuse the first hierarchy separator character that is discovered in a
           user-given mailbox name.

     imap-keepalive-USER@HOST, imap-keepalive-HOST, imap-keepalive
           IMAP servers may close the connection after a period of inactivity; the standard
           requires this to be at least 30 minutes, but practical experience may vary.  Setting
           this variable to a numeric `value' greater than 0 causes a `NOOP' command to be sent
           each `value' seconds if no other operation is performed.

     imap-list-depth
           When retrieving the list of folders on an IMAP server, the folders command stops after
           it has reached a certain depth to avoid possible infinite loops.  The value of this
           variable sets the maximum depth allowed.  The default is 2.  If the folder separator
           on the current IMAP server is a slash `/', this variable has no effect and the folders
           command does not descend to subfolders.

     imap-use-starttls-USER@HOST, imap-use-starttls-HOST, imap-use-starttls
           Causes S-nail to issue a `STARTTLS' command to make an unencrypted IMAP session TLS
           encrypted.  This functionality is not supported by all servers, and is not used if the
           session is already encrypted by the IMAPS method.

SEE ALSO

     bogofilter(1), gpg(1), more(1), newaliases(1), openssl(1), sendmail(1), sh(1),
     spamassassin(1), iconv(3), setlocale(3), aliases(5), termcap(5), terminfo(5), locale(7),
     mailaddr(7), re_format(7), mailwrapper(8), sendmail(8)

HISTORY

     M. Douglas McIlroy writes in his article “A Research UNIX Reader: Annotated Excerpts from
     the Programmer's Manual, 1971-1986” that a mail(1) command already appeared in First Edition
     UNIX in 1971:

           Electronic mail was there from the start.  Never satisfied with its exact behavior,
           everybody touched it at one time or another: to assure the safety of simultaneous
           access, to improve privacy, to survive crashes, to exploit uucp, to screen out foreign
           freeloaders, or whatever.  Not until v7 did the interface change (Thompson).  Later,
           as mail became global in its reach, Dave Presotto took charge and brought order to
           communications with a grab-bag of external networks (v8).

     BSD Mail was written in 1978 by Kurt Shoens and developed as part of the BSD UNIX
     distribution until 1995.  Mail has then seen further development in open source BSD
     variants, noticeably by Christos Zoulas in NetBSD.  Based upon this Nail, later Heirloom
     Mailx, was developed by Gunnar Ritter in the years 2000 until 2008.  Since 2012 S-nail is
     maintained by Steffen (Daode) Nurpmeso.  This man page is derived from “The Mail Reference
     Manual” that was originally written by Kurt Shoens.

AUTHORS

     Kurt Shoens, Edward Wang, Keith Bostic, Christos Zoulas, Gunnar Ritter.  S-nail is developed
     by Steffen Nurpmeso <s-mailx@lists.sdaoden.eu>.

CAVEATS

     [v15 behaviour may differ] Interrupting an operation via SIGINT aka ‘control-C’ from
     anywhere else but a command prompt is very problematic and likely to leave the program in an
     undefined state: many library functions cannot deal with the siglongjmp(3) that this
     software (still) performs; even though efforts have been taken to address this, no sooner
     but in v15 it will have been worked out: interruptions have not been disabled in order to
     allow forceful breakage of hanging network connections, for example (all this is unrelated
     to ignore).

     The SMTP and POP3 protocol support of S-nail is very basic.  Also, if it fails to contact
     its upstream SMTP server, it will not make further attempts to transfer the message at a
     later time (setting save and sendwait may be useful).  If this is a concern, it might be
     better to set up a local SMTP server that is capable of message queuing.

BUGS

     After deleting some message of a POP3 mailbox the header summary falsely claims that there
     are no messages to display, one needs to perform a scroll or dot movement to restore proper
     state.  In ‘thread’ed sort mode a power user may encounter crashes very occasionally (this
     is may and very).  The file TODO in the source repository lists future directions.

     Please report bugs to the contact-mail address, e.g., from within s-nail: ‘? eval mail
     $contact-mail’.  Including the verbose output of the command version may be helpful, e.g.,

           ? wysh set escape=! verbose; vput version xy; unset verbose;\
             eval mail $contact-mail
           Bug subject
           !I xy
           !.

     Information on the web at ‘$ s-nail -X 'echo $contact-web' -Xx’.