xenial (1) heirloom-mailx.1.gz

Provided by: s-nail_14.8.6-1_amd64 bug

NAME

     S-nail [v14.8.6] — send and receive Internet mail

SYNOPSIS

     s-nail -h | --help
     s-nail [-BDdEFintv~] [-A account] [-a attachment] [-b bcc-addr] [-c cc-addr] [-q quote-file] [-r from-addr]
            [-S variable[=value]] [-s subject] [-X cmd] [-.] to-addr ... [-- mta-option ...]
     s-nail [-BDdEeHiNnRv~#] [-A account] [-L spec-list] [-r from-addr] [-S variable[=value]] [-X cmd] -f [file]
            [-- mta-option ...]
     s-nail [-BDdEeHiNnRv~#] [-A account] [-L spec-list] [-r from-addr] [-S variable[=value]] [-u user] [-X cmd]
            [-- mta-option ...]

DESCRIPTION

           Compatibility note: S-nail and part of its configuration syntax will change  in  v15.0.   Until  then
           there  will  exist  a  partial but growing number of backward and forward compatibility configuration
           options.  To choose the new syntax and behaviour already today, the binary option v15-compat must  be
           set.   The  manual  will refer to it via [v15-compat] and [no v15-compat] as necessary.  Commands and
           variables which will vanish in v15.0 are marked [Obsolete].

     S-nail is a mail processing system with a command syntax reminiscent of ed(1) with lines replaced by
     messages.  It is intended to provide the functionality of the POSIX mailx(1) command and offers (mostly
     optional) extensions for line editing, IDNA, MIME, S/MIME, SMTP and POP3 (and IMAP).  It is usable as a
     mail batch language.

   Options
     -A account  Executes an account command (see below) for account after the startup files have been read.

     -a file     Attach the given file to the message.  The same filename conventions as described in the
                 section COMMANDS apply: shell word expansion is restricted to the tilde ‘~’ and variables.
                 Shall file not be accessible but contain a ‘=’ character, then anything after the ‘=’ is
                 assumed to specify the input character set and anything before ‘=’ the filename: this is the
                 only option to specify the input character set (and don't perform any character set conversion)
                 for text attachments from the command line, not using the ~@ tilde escape command.

     -B          Make standard input and standard output line-buffered.

     -b address  Send blind carbon copies to the given list of addresses.  Sending mail below goes into more
                 detail on that.

     -c address  Send carbon copies to the given list of addresses.

     -D          [Option] Set the disconnected variable.

     -d          Set the debug variable, which enables debug messages and disables message delivery.

     -E          Set the skipemptybody variable and thus discard messages with an empty message part body.  This
                 is useful for sending messages from scripts.

     -e          Just check if mail is present in the system mailbox.  If yes, return an exit status of zero, a
                 non-zero value otherwise.

     -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 [file]   Read in the contents of the user's 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).  Some
                 special conventions are recognized for the string file which are documented for the file
                 command below.  Note that file is not a direct 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, prefix it with a (relative) path, as in ‘./-hyphenbox.mbox’.

     -H          Print a header summary of all messages and exit.  A configurable summary view is available via
                 the -L option.

     -h          Print a short usage summary.  Because of widespread use a --help argument will have the same
                 effect.

     -i          Set the ignore variable to ignore tty interrupt signals.

     -L spec-list
                 Print a header summary of only those messages that match the given spec-list, then exit.  See
                 the section Specifying messages for the format of spec-list.  If the -H option has been given
                 in addition no header summary is produced, but S-nail will instead indicate via its exit status
                 wether spec-list 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).

     -N          Unset the header variable and thus inhibit initial display of message headers when reading mail
                 or editing a mail folder.

     -n          Inhibits reading s-nail.rc upon startup.  This option should be activated for S-nail scripts
                 that are invoked on more than one machine, because the contents of that file may differ between
                 them.  (The same behaviour can be achieved by setting the NAIL_NO_SYSTEM_RC environment
                 variable.)

     -q file     Start the message with the contents of the specified file.  May be given in send mode only.

     -R          Any folder opened will be in read-only mode.

     -r address  If address is a valid address then it specifies the envelope sender address to be passed to the
                 MTA as ‘-f address’ when a message is send.  Shall address include a user name, comments etc.,
                 then the components will be separated and the name part will be passed to the MTA individually
                 via ‘-F name’.  The given address will also be assigned to the from variable, just as if
                 additionally ‘-Sfrom=address’ had been specified (therefore affecting SMTP data transfer, too).

                 If instead an empty string is passed as address then the content of the variable from will be
                 evaluated and used for this purpose whenever the MTA is contacted.  Note that S-nail by
                 default, without -r that is, neither passes ‘-f’ nor ‘-F’ flags to the MTA by itself.

     -S variable[=value]
                 Sets the internal option variable and, in case of a value option, assigns value to it.  Even
                 though options set via -S may be overwritten from within resource files, the command line
                 setting will be reestablished after all resource files have been loaded.

     -s subject  Specify the subject of the to-be-sent message.

     -t          The message to be sent is expected to contain a message header with ‘To:’, ‘Cc:’, or ‘Bcc:’
                 fields giving its recipients, which will be added to those given on the command line.  If a
                 message subject is specified via ‘Subject:’ then it'll be used in favour of one given on the
                 command line.

                 Also understood are ‘Reply-To:’ (possibly overriding replyto), ‘Sender:’ (p. o. sender),
                 ‘Organization:’ (p. o. ORGANIZATION).  Note you can also specify ‘From:’, possibly overriding
                 from and the envelope address possibly specified with the option -r!

                 The following, which are normally created automatically based upon the message context, can
                 also be specified: ‘Message-ID:’, ‘In-Reply-To:’, ‘References:’ and ‘Mail-Followup-To:’
                 (special address massage will however still occur for the latter).

     -u user     Read the system mailbox of user (appropriate privileges presumed), and “assume to be” user in
                 some aspects, e.g. in respect to file–expansions of ‘%’ etc.; also see USER.

     -V          Print S-nail's version and exit.

     -v          Setting the verbose option causes some verbosity (like printing of certificate chains).  Using
                 it twice increases the level of verbosity.

     -X cmd      Add the given cmd to the list of commands to be executed before normal operation starts.
                 Correlates with -# and batch-exit-on-error; the only possibility to execute commands in non-
                 interactive mode when reading startup files is actively prohibited.

     -~          Enable TILDE ESCAPES even if not in interactive mode.

     -#          This sets several options to prepare S-nail for working in (most likely non-interactive) batch
                 mode: dot, emptystart, noheader, quiet, sendwait, as well as MBOX and folder (both to
                 /dev/null).  It also enables processing of TILDE ESCAPES.  E.g., the following should send an
                 email message to “bob”:

                       $ LC_ALL=C printf 'm bob\n~s ubject\nText\n.\nx\n' | \
                         LC_ALL=C MAILRC=/dev/null s-nail -n -# -Snosave

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

     In the above list of supported command line options, -D, -d, -E, -i, -N and -v are implemented by means of
     setting the respective option, as via -S.  [mta-option ...] arguments that are given at the end of the
     command line after a ‘--’ separator will be passed through to the mail-transfer-agent (MTA) and persist for
     an entire (interactive) session – if the setting of expandargv allows their recognition; MTA arguments can
     also be specified in the variable sendmail-arguments; find MTA interaction described in more detail in the
     documentation of sendmail.  MTA arguments are ignored when mail is send via SMTP data transfer.

   A starter
     S-nail is a direct descendant of the BSD Mail program that was introduced in 1978 (itself superceeding the
     simpler UNIX mail program) and used to introduce itself (in the Mail reference manual) as follows:

           Mail provides a simple and friendly environment for sending and receiving mail.  It divides incoming
           mail into its constituent messages and allows the user to deal with them in any order.  In addition,
           it provides a set of ed(1)-like commands for manipulating messages and sending mail.  Mail offers the
           user simple editing capabilities to ease the composition of outgoing messages, as well as providing
           the ability to define and send to names which address groups of users.

     S-nail is thus the user side of the Unix mail system, whereas the system side (mail-transfer-agent, MTA)
     was traditionally taken by sendmail(8); today postfix(1) or exim(8) are often used for this purpose
     instead.  If the [Option]al SMTP feature has been built into 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.  The default global s-nail.rc file already
     bends those standard settings a bit towards more user friendliness and safety, e.g., it sets the hold and
     keepsave options in order to suppress the automatic moving of messages to MBOX that would otherwise occur
     (see Message states) and keep to not remove empty files in order not to mangle file permissions when files
     eventually get recreated.  It does not set the folder option so that by default file grouping (via the ‘+’
     prefix as documented also for file) is not enabled.  The section EXAMPLES contains some further
     suggestions.

   Sending mail
     To send a message to one or more people, using a local mail-transfer-agent (MTA; the executable path can be
     set via sendmail) or the [Option]al builtin SMTP (set and see the variable smtp) 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:

           $ s-nail -s Subject -a attachm.ent bill@host1 'Bob <bob@host2>'
           # But... try it in an isolated dry-run mode first
           $ LC_ALL=C MAILRC=/dev/null \
             s-nail -n -d -vv -Sfrom="me <he@re>" \
               -s Subject -. "(Lovely) Bob <bob@host2>"

     The user is then expected to type in the message contents.  In this compose mode S-nail treats lines
     beginning with the character ‘~’ special – these are so-called TILDE ESCAPES which can be used to read in
     files, process shell commands, add and edit attachments and more; e.g., the tilde escape ‘~e’ will start
     the text editor to revise the message in it's current state, ‘~h’ allows editing of the message recipients
     and ‘~?’ gives an overview of available tilde escapes.  Typing control-D ‘^D’ at the beginning of an empty
     line leaves compose mode and causes the message to be send, whereas typing control-C ‘^C’ twice will abort
     the current letter (saving its contents in the file denoted by DEAD unless nosave is set.)

     A number of options can be used to alter default behavior; e.g., setting (also via -S) editalong will
     automatically startup a text editor when compose mode is entered, askcc will cause the user to be prompted
     actively for carbon-copy recipients and the dot option will allow leaving compose mode by writing a line
     consisting solely of a dot (‘.’).

     Very important, though, is to define which Character sets may be used when sending messages, usually by
     setting the option sendcharsets accordingly, having read the section The mime.types files to understand how
     the MIME-type of outgoing attachments is classified and the knowledge that messages are sent asynchronously
     unless sendwait is set: only with it MTA delivery errors will be recognizable.

     Setting from is often necessary (e.g., in conjunction with smtp) or desirable, you may want to do some dry-
     run tests before you go.  Saving a copy of the sent messages in a record may also be desirable – as for
     most mailbox file targets some special conventions are recognized, see the file command for more on that.
     The section On URL syntax and credential lookup will spread some light on the ‘USER@HOST’ variable chains
     as well as on using URLs for accessing protocol-specific resources, like smtp, and EXAMPLES contains an
     example configuration for sending messages via some of the well-known public mail providers; note it also
     gives a compact overview on how to setup a secure SSL/TLS environment.

     Message recipients (as specified on the command line or defined in ‘To:’, ‘Cc:’ or ‘Bcc:’) may not only be
     email addressees but can also be names of mailboxes and even complete shell command pipe specifications.
     Proper quoting may be necessary, e.g., to embed whitespace characters.  (Recall that S-nail deals with mail
     standards, therefore those define the rules with which content is interpreted.)  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.

     If the variable expandaddr is set then extended recipient addresses will optionally be accepted: Any name
     which 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 starts with the
     character slash ‘/’ or the character sequence dot slash ‘./’ is treated as a file, regardless of the
     remaining content.  Any other name which contains an at sign ‘@’ character is treated as a network address;
     Any other name which starts with a plus sign ‘+’ character specifies a mailbox name; Any other name which
     contains a slash ‘/’ character but no exclamation mark ‘!’ or percent sign ‘%’ character before also
     specifies 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 MAILRC=/dev/null \
               s-nail -n -Sv15-compat -Ssendwait -Snosave \
                 -Sexpandaddr=fail,-all,+addr -s test \
                 -. bob@home.net

     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:

           alias cohorts bill jkf mark kridle@ucbcory ~/mail/cohorts.mbox

     Please note that this mechanism has nothing in common with the system wide aliases that may be used by the
     local MTA (mail-transfer-agent), which are subject to the ‘name’ constraint of expandaddr and are often
     tracked in a file /etc/aliases (and documented in aliases(5) and sendmail(1)).  Personal aliases will be
     expanded by S-nail before the message is sent, and are thus a convenient alternative to specifying each
     addressee by itself.

     To avoid environmental noise scripts should “detach” s-nail from any configuration files and create a
     script-local environment, either by pointing the MAILRC variable to a custom configuration file, or by
     using the -S command line option to specify options:

           $ env LC_ALL=C MAILRC=/dev/null password=NOTSECRET \
             s-nail -n -Sv15-compat -Ssendwait -Snosave \
               -Sexpandaddr=fail,-all,+addr \
               -S 'smtp=smtps://mylogin@some.host:465' -Ssmtp-auth=login \
               -S 'from=scriptreply@domain' \
               -s 'subject' -a attachment_file \
               -. "Recipient 1 <recipient1@domain>" recipient2@domain \
               < content_file

   Reading mail
     When invoked without addressees S-nail enters interactive mode in which mails may be read.  When used like
     that the user's system mailbox is read in and a one line header of each message therein is printed.  (See
     the command file for an in-depth description of the different mailbox types that exist.)  Note that if the
     mailbox is empty S-nail will exit after printing a message unless the option emptystart is found to be set.

     At the prompt the command list will give a listing of all available commands and help will give a summary
     of some common ones.  If the [Option]al documentation strings are available one can type ‘?X’ and see the
     actual expansion of ‘X’ and what it's purpose is, i.e., commands can be abbreviated (note that POSIX
     defines some abbreviations, so that the alphabetical order of commands doesn't necessarily relate to the
     abbreviations; it is possible to define overwrites with the ghost command, however).

     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 option showlast will instead cause usage of the last message for this purpose.

     Messages can be printed with the print command, or short: ‘p’.  By default the current message (“dot”) is
     printed, but like with most other commands it is possible to give a fancy message specification (see
     Specifying messages), e.g., ‘p:u’ will display all unread messages, ‘p.’ will print the “dot”, ‘p 1 5’ will
     print the messages 1 and 5, ‘p 1-5’ will print the messages 1 through 5, and ‘p-’ and ‘p+’ will print the
     last and the next message, respectively.

     In the default setup all header fields of a message will be printed, but this can be changed: either by
     blacklisting a list of fields via ignore, or by whitelisting only a given list with the retain command,
     e.g., ‘retain date from to cc subject’.  In order to print all header fields of a message regardless of
     currently active ignore or retain lists, use the command Print.  The variable crt controls wether and when
     S-nail will use the configured PAGER for printing instead of directly writing to the terminal (generally
     speaking).

     Dependent upon the configuration a Command line editor aims at making user experience with the many
     COMMANDS a bit nicer.  When reading the system mailbox or when -f (or file) specified a mailbox explicitly
     prefixed with the special ‘%:’ modifier then messages which have been read will be moved to the user's MBOX
     file automatically when the mailbox is left, either by changing the active mailbox or by quitting S-nail
     (also see Message states).

     After examining a message the user can also delete ‘d’ the message, reply ‘r’ to the sender and all
     recipients or Reply ‘R’ exclusively to the sender.  Messages can also be forwarded (shorter alias is fwd).
     Note that when replying to or forwarding a message recipient addresses will be stripped from comments and
     names unless the option fullnames is set.  Deletion causes S-nail to forget about the message; This is not
     irreversible, though, one can undelete ‘u’ the message by giving its number, or the S-nail session can be
     ended by giving the exit ‘x’ command.

     To end a mail processing session one may either issue quit ‘q’ to cause a full program exit, which possibly
     includes automatic moving of read messages to MBOX as well as updating the [Option]al command line editor
     history file, or use the command exit ‘x’ instead in order to prevent any of these actions.

   Viewing HTML mail and MIME attachments
     Messages which are HTML-only get more and more common and of course many messages come bundled with a
     bouquet of MIME attachments.  Whereas S-nail [Option]ally supports a simple HTML-to-text converter to deal
     with HTML messages (see The mime.types files), it normally can't deal with any of these itself, but 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 display the content on the
     terminal, or display the content themselves, for example in a graphical window.  The latter type of
     programs by default “blocks” S-nail until the external viewer has terminated, but asynchronous side-by-side
     execution is also possible, in which case S-nail will continue to display the message and remain
     responsive.

     To install an external handler program for a specific MIME type set an according pipe-TYPE/SUBTYPE
     variable.  To define a handler for a specific file extension set the respective pipe-EXTENSION variable –
     these handlers take precedence.  The variable mime-counter-evidence can be set to improve dealing with
     faulty MIME part declarations as are often seen in real-life messages.  E.g., to display a HTML message
     inline (that is, converted to a more fancy plain text representation than the builtin converter 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:

           if $features !@ HTML-FILTER
             #set pipe-text/html="elinks -force-html -dump 1"
             set pipe-text/html="lynx -stdin -dump -force_html"
             # Display HTML as plain text instead
             #set pipe-text/html=@
           endif
           mimetype '@ application/mathml+xml mathml'
           set pipe-application/pdf="@&set -C;\
               : > \"${TMPDIR}/${NAIL_FILENAME_GENERATED}\"; \
               trap \"rm -f \\\"${TMPDIR}/${NAIL_FILENAME_GENERATED}\\\"\" \
                 EXIT INT QUIT PIPE TERM;\
               set +C;\
               cat > \"${TMPDIR}/${NAIL_FILENAME_GENERATED}\"; \
               xpdf \"${TMPDIR}/${NAIL_FILENAME_GENERATED}\""

     Note: special care must be taken when using such commands as mail viruses may be distributed by this
     method: if messages of type ‘application/x-sh’ or files with the extension ‘.sh’ were blindly filtered
     through the shell, for example, a message sender could easily execute arbitrary code on the system S-nail
     is running on.  For more on MIME, also in respect to sending of messages, see the section The mime.types
     files and the command mimetype.

   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 doesn't unmlist automatically, but only resets the
     subscription attribute.)  Using the commands without arguments will print out (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 header display.

     [Option]ally mailing lists may also be specified as regular expressions, 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
           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 wether 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 sender 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-To:’ 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, dependend on the variable reply-to-honour, use an also set
     ‘Reply-To:’ for this purpose in order to accept a list administrators' wish that is supposed to have been
     manifested like that (but only if it provides a single address which resides on the same domain as what is
     stated in ‘List-To:’).

   Resource files
     Upon startup S-nail reads in several resource files:

     s-nail.rc      System wide initialization file.  Reading of this file can be suppressed, either by using
                    the -n command line option, or by setting the environment variable NAIL_NO_SYSTEM_RC.

     ~/.mailrc      File giving initial commands.  A different file can be chosen by setting the environment
                    variable MAILRC.

     NAIL_EXTRA_RC  Can be used to define an optional startup file to be read after ~/.mailrc.  This variable is
                    only honoured in certain circumstances (see its documentation for more).

     The content of these files is interpreted as follows:

        A lines' leading whitespace is ignored.
        An empty line is ignored.
        If the line (content) starts with the number sign ‘#’ then it is a comment-command – a real command! –
         and also ignored.  This command is the only form of comment that is understood.
        A line can be “continued” on the next line if the newline character is “escaped” by preceding it with
         the backslash character ‘\’.  Note that any leading whitespace of follow lines is removed: If
         whitespace is desired it must be placed before the backslash.  For example:

                # This line is a comment command.  And y\
                   es, it is really continued here.
               set debug \
                   verbose

   Character sets
     [Option] S-nail detects the character set of the terminal by using mechanisms that are controlled by the
     LC_CTYPE locale setting (the manual for setlocale(3) should give an overview); the S-nail internal variable
     ttycharset will be set to the detected terminal character set accordingly and will thus show up in the
     output of the commands set and varshow.

     However, a user supplied ttycharset value is not overwritten by this detection mechanism: this “feature”
     must be used if the detection doesn't work properly, and it may be used to adjust the name of the locale
     character set.  E.g., on BSD systems one may use a locale with the character set ISO8859-1, which is not a
     valid name for this character set; to be on the safe side, one may set ttycharset to the correct name,
     which is ISO-8859-1.

     Note that changing the value doesn't mean much beside that, since several aspects of the real character set
     are implied by the locale environment of the system, and that stays unaffected by the content of an
     overwritten ttycharset variable.  (This is mostly an issue when interactively using S-nail, though.  It is
     actually possible to send mail in a completely “faked” locale environment.)

     If no character set conversion capabilities have been compiled into S-nail (i.e., no iconv(3) library has
     been found), then ttycharset will be the only supported character set, it is simply assumed that it can be
     used to exchange 8-bit messages, 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 the
     mentioned ISO-8859-1.

     When reading messages, their text is converted into ttycharset as necessary in order to display them on the
     users terminal.  Unprintable characters and invalid byte sequences are detected and replaced by proper
     substitution characters (unless the variable print-all-chars was set once S-nail was started).  Also see
     charset-unknown-8bit to deal with another hairy aspect of message interpretation.

     When sending messages all 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 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.  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 sent and its text will be saved to DEAD.  In general, if the
     message “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 a 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.

   Message states
     S-nail differentiates in between several different message states; the current state will be reflected in
     the header display if headline is configured to do so.  In S-nail message states are inspected when leaving
     a mailbox and may cause messages to be automatically moved to the special MBOX mailbox – because this may
     be irritating to users which are used to “more modern” mail-user-agents, the default global s-nail.rc sets
     the 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 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 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 delete, dp, and dt commands may also
                  cause the next message to be marked as read, depending on the value of the autoprint variable.
                  Except when the exit command is used, messages that are in the system mailbox or in mailboxes
                  which were opened with the special ‘%:’ prefix and are in ‘read’ state when the mailbox is
                  left will be saved in MBOX unless the option 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 the system mailbox or in mailboxes which were
                  opened with the special ‘%:’ prefix and are in ‘saved’ state when the mailbox is left will be
                  deleted; they will be saved in MBOX when the option keepsave is set.

   Specifying messages
     Commands such as print 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 header summary.  Multiple colon modifiers can be joined into one, e.g., ‘:du’.  The following
     special message names exist:

     :n        All ‘new’ messages.

     :o        All old messages (any not in state ‘read’ or ‘new’).

     :u        All ‘unread’ messages.

     :d        All ‘deleted’ messages (for the undelete command).

     :r        All ‘read’ messages.

     :f        All flagged messages.

     :a        All answered messages (cf. the markanswered variable).

     :t        All messages marked as draft.

     :s        [Option] All messages classified as spam.

     :S        [Option] All messages with unsure spam classification.

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

     ;         The message that was previously the current message.

     ,         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 next previous undeleted message, or the next previous deleted message for the undelete
               command.  In sorted/threaded mode, the next previous such message in the sorted/threaded order.

     +         The next undeleted message, or the next deleted message for the undelete command.  In
               sorted/threaded mode, the next such message in the sorted/threaded order.

     ^         The first undeleted message, or the first deleted message for the undelete command.  In
               sorted/threaded mode, the first such message in the sorted/threaded order.

     $         The last message.  In sorted/threaded mode, the last message in the sorted/threaded order.

     &x        In threaded 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 for the previous command.

     /string   All messages that contain string in the subject field (case ignored).  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 (see re_format(7)) support is available expr will be interpreted as one if any of the
               “magical” regular expression characters is seen.  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, as in

                     '@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.  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.  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 ‘=’ can
               be used to perform full text searches – whereas the former searches only the body, the latter
               also searches the message header.

               This message specification performs full text comparison, but even with regular expression
               support it is almost impossible to write a search expression that savely matches only a specific
               address domain.  To request that the 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 header name (abbreviation) with a tilde ‘~’:

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

     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; the
               [Option]al IMAP-style ‘(from address)’ expression can also be used if substring matches are
               desired.

     [Option] IMAP-style SEARCH expressions may also be used.  This addressing mode is available with all types
     of folders; S-nail will perform the search locally as necessary.  Strings must be enclosed by double quotes
     ‘"’ in their entirety if they contain white space or parentheses; within the quotes, only backslash ‘\’ 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 URL syntax and credential lookup
     [v15-compat] For accessing protocol-specific resources, like POP3 mailboxes, usage of compact and
     standardized 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 IMAP protocol but not by POP3.

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

     If ‘USER’ and ‘PASSWORD’ are specified as part of an URL they must be given in URL percent encoded (RFC
     3986) form – the command urlencode can be used to perform the encoding and show the encoded value.  (This
     doesn't really conform to any standard, but for one it isn't used for any data exchange over the internet,
     and second it's easier for users to simply call urlencode on a string and use that instead of having to
     deal with several different standards.)  On the other hand, values given in variables are expected not to
     be URL percent encoded.

     Many variable options 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’ isn't 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, wether it came from an URL or
     not.

     For example, wether an hypothetical URL ‘smtp://hey%3Ayou@our.house’ had been given that includes a user,
     or wether 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 wether ‘smtp-use-starttls-hey:you@our.house’ is defined,
     then wether ‘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 informations 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 users .netrc file 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:
         either the name that has been given with the -u command line option (or, equivalently, but with less
         precedence, the environment variable USER), or the “current” user.  The identity of this user 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 – it should be noted once that specifying the password in
         the URL is only syntactic sugar for the user, it'll never be part of an URL that S-nail uses itself –,
         then if the ‘USER’ has been found through the [Option]al .netrc file lookup then that may have already
         provided the password, too.  Otherwise the variable chain password-USER@HOST, password-HOST, password
         is looked up.

         [Option] Then if any of the variables of the chain agent-shell-lookup-USER@HOST,
         agent-shell-lookup-HOST, agent-shell-lookup is set the shell command specified therein is run and the
         output (less newline characters) will be used as the password.  It is perfectly valid for such an agent
         to simply not return any data, in which case the password lookup is continued somewhere else; Any
         command failure is treated as a hard error, however.

         The next variable chain that is inspected is the [Option]al netrc-lookup-USER@HOST, netrc-lookup-HOST,
         netrc-lookup, but this time looking only for the password (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-message-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 smtp=smtp://USER:PASS@HOST smtp-use-starttls \
               smime-sign smime-sign-cert=+smime.pair

     The section EXAMPLES contains complete example configurations.

   Command line editor
     [Option] S-nail can be configured to support a command line editor and command history lists which are
     saved in between sessions.  One may link against fully-fledged external libraries (readline(6),
     editline(3)) or use S-nail's own command line editor NCL (Nail-Command-Line) instead, which should work in
     all environments which comply to the ISO C standard (ISO/IEC 9899:1990/Amendment 1:1995).  When an external
     library is used, interactive behaviour of S-nail relies on that library and may not correspond one-to-one
     to what is described in this manual.

     Regardless of the actually used command line editor history entries will be created for lines entered in
     command mode only, and creation of such an entry can be forcefully suppressed by starting the line with a
     space character.  Note that history handling is by itself an optional feature and may therefore not be
     available.  For more information see the documentation of the options emptystart, history-gabby,
     history-gabby-persist, line-editor-disable, NAIL_HISTFILE and NAIL_HISTSIZE.

     The builtin S-nail command line editor supports the following operations; the notation ‘^-character’ stands
     for the combination of the “Control” key plus the mentioned character, e.g., ‘^A’ means “hold down control
     key and press the A key”:

     ‘^A’   Go to the start of the line.
     ‘^B’   Move the cursor backward one character.
     ‘^D’   Forward delete the character under the cursor; quits S-nail if used on the empty line unless the
            ignoreeof option is set.
     ‘^E’   Go to the end of the line.
     ‘^F’   Move the cursor forward one character.
     ‘^G’   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.  In all cases S-nail will reset a possibly used multibyte
            character input state machine.
     ‘^H’   The same as “backspace”: backward delete one character.
     ‘^I’   [Option] The same as “horizontal tabulator”: try to expand the word before the cursor.  (Note this
            is not “tabulator-completion” as is known from the sh(1) but really means the usual S-nail
            expansion, as documented for file, yet it involves shell expansion as a last step, too.)
     ‘^J’   The same as “ENTER”: complete this line of input.
     ‘^K’   Delete all characters from the cursor to the end of the line.
     ‘^L’   Repaint the line.
     ‘^N’   [Option] Go to the next history entry.
     ‘^O’   Execute the command dp.
     ‘^P’   [Option] Go to the previous history entry.
     ‘^R’   [Option] Complete the current line from (the remaining older) history entries.
     ‘^U’   The same as ‘^A’ followed by ‘^K’.
     ‘^W’   Delete the characters from the one preceding the cursor to the preceding word boundary.
     ‘^X’   Move the cursor forward one word boundary.
     ‘^Y’   Move the cursor backward one word boundary.

     If problems with commands that are based upon rightwise movement are encountered, adjustments of the option
     line-editor-cursor-right may solve the problem, as documented for it.

     If the terminal produces key sequences which are compatible with xterm(1) then the left and right cursor
     keys will map to ‘^B’ and ‘^F’, respectively, the up and down cursor keys will map to ‘^P’ and ‘^N’, and
     the Home/End/PgUp/PgDown keys will call the z command with the respective arguments ‘0’, ‘$’, ‘-’ and ‘+’
     (i.e., perform scrolling through the header summary list).

   Coloured message display
     [Option] S-nail can be configured to support coloured message display, realized by emitting ANSI colour
     escape sequences.  Colours are only used when the TERM environment variable is set and either the terminal
     type can be found in colour-terms or its name includes the string ‘color’.

     On top of that the binary option colour-pager defines wether these colour sequences are also generated when
     the output of a command needs to go through the 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 colour sequences, please see the option for more details.

     To forcefully disable all colour support, set colour-disable.

     Colours can be configured through font attributes (‘ft=’ – ‘bold’, ‘invers’ and ‘underline’), foreground
     (‘fg=’) and background (‘bg=’) colours (‘black’, ‘blue’, ‘green’, ‘red’, ‘brown’, ‘magenta’, ‘cyan’ and
     ‘white’).  Multiple specifications can be joined in a comma separated list, as in

           set colour-msginfo="ft=bold,fg=magenta,bg=cyan"

     Options to be set are colour-msginfo, colour-partinfo, colour-from_, colour-header and colour-uheader, as
     well as colour-user-headers, which is a list of headers to be colourized via colour-uheader instead of the
     default colour-header.

COMMANDS

     Each command is typed on a line by itself, and may take arguments following the command word.  The command
     need not be typed in its entirety – the first command which matches the typed prefix is used.  (The command
     list prints a sorted list of available commands, and the command help (or ?), when given an argument, will
     show a documentation string for the expansion, as in ‘?unc’; documentation strings are however [Option]al.)

     For commands which take message lists as arguments, the next message forward that satisfies the command's
     requirements will be used shall no explicit message list have been passed.  If there are no messages
     forward of the current message, the search proceeds backwards, and if there are no good messages at all,
     S-nail types “no applicable messages” and aborts the command.  The arguments to commands can be quoted,
     using the following methods:

              An argument can be enclosed between paired double-quotes ‘"argument"’ or single-quotes
               ‘'argument'’; any white space, shell word expansion, or backslash 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 backslash ‘\’, as in
               ‘"y\"ou"’.
              An argument that is not enclosed in quotes, as above, can usually still contain space characters
               if those spaces are backslash-escaped.
              A backslash outside of the enclosing quotes is discarded and the following character is treated
               literally as part of the argument.
              An unquoted backslash at the end of a command line is discarded and the next line continues the
               command.

     Filenames, where expected, are subsequently subjected to the following transformations, in sequence:

              If the filename begins with an unquoted plus sign, and the folder variable is defined, the plus
               sign will be replaced by the value of the folder variable followed by a slash.  If the folder
               variable is unset or is set to null, the filename will be unchanged.
              Shell word expansions are applied to the filename.  Compatibility note: on the long run support
               for complete shell word expansion will be replaced by an internally implemented restricted
               expansion mechanism in order to circumvent possible security impacts through shell expansion.
               Expect that a growing number of program parts only support this restricted syntax:

               Meta expansions are applied to the filename: leading tilde characters (‘~/’) will be replaced by
               the expansion of HOME, and any occurrence of ‘$VARIABLE’ (or ‘${VARIABLE}’) will be replaced by
               the expansion of the variable, if possible; S-nail internal as well as environmental (shell)
               variables can be accessed through this mechanism.  In order to include a raw ‘$’ character
               precede it with a backslash ‘\’, to include a backslash double it.  If more than a single
               pathname results from this expansion and the command is expecting one file, an error results.

               Note that in interactive display context, in order to allow simple value acceptance (typing
               “ENTER”), backslash quoting is performed automatically as necessary, e.g., a file ‘diet\ is
               \curd.txt’ will be displayed as ‘diet\\ is \\curd.txt’.

     The following commands are available:

     #        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.

     ~        Interprets the remainder of the word as a macro name and passes it through to the call command;
              e.g., ‘~mymacro’ is a shorter synonym for ‘call mymacro’.

     -        Print out the preceding message.  If given a numeric argument n, goes to the n'th previous message
              and prints it.

     =        Show the current message number (the “dot”).

     ?        Prints a brief summary of commands.  [Option] Given an argument a synopsis for the command in
              question is printed 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 display changes.

     !        Executes the SHELL (see sh(1)) command which follows.

     |        A synonym for the pipe command.

     account  (ac) Creates, selects or lists (an) account(s).  An account is a group of commands and variable
              settings which together usually arrange the environment for the purpose of creating a system
              login.  Without any argument a listing of all defined accounts and their content is printed.  A
              specific account can be activated by giving solely its name, resulting in the system mailbox or
              inbox of that account to be activated as via an explicit use of the command file.  The two
              argument form is identical to defining a macro as via define.  For example:

                    account myisp {
                      set folder=imaps://mylogin@imap.myisp.example
                      set record=+Sent
                      set from="myname@myisp.example (My Name)"
                      set smtp=smtp://mylogin@smtp.myisp.example
                    }

              creates an account named ‘myisp’ which can later be selected by specifying ‘account myisp’.  The
              special account ‘null’ (case-insensitive) always exists.  localopts can be used to localize
              account settings – different to normal macros the settings will be reverted once the account is
              switched off.  Accounts can be deleted via unaccount.

     alias    (a) With no arguments, prints out all currently-defined aliases.  With one argument, prints out
              that alias.  With more than one argument, creates a new alias or appends to an existing one.
              unalias can be used to delete aliases.

     alternates
              (alt) Manage a list of alternate addresses / names of the active user, members of which will be
              removed from recipient lists when replying to messages (and the metoo variable is not set).  If
              arguments are given the set of alternate names is replaced by them, without arguments the current
              set is displayed.

     answered
              Takes a message list and marks each message as having been answered.  This mark has no technical
              meaning in the mail system; it just causes messages to be marked in the header summary, and makes
              them specially addressable.

     cache    [Option] Only applicable to cached IMAP mailboxes; takes a message list and reads the specified
              messages into the IMAP cache.

     call     Calls a macro that has been created via define.

     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 a message list and a file name 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.

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

     collapse
              Only applicable to threaded mode.  Takes a message list and makes all replies to these messages
              invisible in header summaries, unless they are in state ‘new’.

     connect  [Option] 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.

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

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

     cwd      Print the current working directory.

     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   Without arguments the current list of macros, including their content, is printed, but otherwise a
              macro is defined.  A macro definition is a sequence of commands in the following form:

                    define name {
                      command1
                      command2
                      ...
                      commandN
                    }

              A defined macro can be invoked explicitly by using the call or ~ commands, or implicitly by
              setting the folder-hook or folder-hook-FOLDER variables.  Note that interpretation of localopts
              depends on how (i.e., “as what”: normal macro, folder hook, account switch) the macro is invoked.
              Macros can be deleted via undefine.

     delete   (d) Marks the given message list as ‘deleted’.  Deleted messages will neither be saved in MBOX nor
              will they be available for most other commands.

     discard  (di) Identical to ignore.  Also see retain.

     disconnect
              [Option] (disco) 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.

     dp, dt   Deletes the current message and prints the next message.  If there is no next message, S-nail says
              “at EOF”.

     draft    Takes a message list and marks each given message as a draft.  This mark has no technical meaning
              in the mail system; it just causes messages to be marked in the header summary, and makes them
              specially addressable.

     echo     (ec) Echoes its arguments, resolving special names as documented for the command file.  The escape
              sequences ‘\a’, ‘\b’, ‘\c’, ‘\f’, ‘\n’, ‘\r’, ‘\t’, ‘\v’, ‘\\’ and ‘\0octal-num’ are interpreted
              just as they are by printf(1) (proper quoting provided).

     edit     (e) Point the text editor (as defined in EDITOR) at each message from the given list in turn.
              Modified contents are discarded unless the writebackedited variable is set.

     elif     Part of the if/elif/else/endif conditional — if the condition of a preceeding if was false, check
              the following condition and execute the following block if it evaluates true.

     else     (el) Part of the if/elif/else/endif conditional — if none of the conditions of the preceeding if
              and elif commands was true, the else block is executed.

     endif    (en) Marks the end of an if/elif/else/endif conditional execution block.

     errors   [Option] Since S-nail uses the console as a user interface it can happen that messages scroll by
              too fast to become recognized.  Optionally 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.

     exit     (ex or x) Exit from S-nail without changing the active mailbox and skip any saving of messages in
              MBOX as well as a possibly tracked command line editor history file.

     features
              Print the list of features that have been compiled into S-nail.

     File     (Fi) Like file, but open the mailbox readonly.

     file     (fi) The file command switches to a new mailbox.  Without arguments it prints the complete state
              of the current mailbox.  If an argument is given, it will write out changes (such as deletions)
              the user has made and open a new mailbox.  Some special conventions are recognized for the name
              argument:

                    #           (number sign) means the previous file,
                    %           (percent sign) means the invoking user's system mailbox (or the value of folder
                                for IMAP folders),
                    %user       means the system mailbox of user (and never the value of folder, regardless of
                                its actual setting),
                    &           (ampersand) means the invoking user's MBOX file and
                    +file       means a file in the folder directory.
                    %:filespec  expands to the same value as filespec, but the file is handled as a 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.

              If the name matches one of the strings defined with the command shortcut, it is replaced by its
              long form and expanded.  If the name ends with ‘.gz’, ‘.bz2’ or ‘.xz’ it is treated as being
              compressed with gzip(1), bzip2(1) or xz(1), respectively, and transparently handled through an
              intermediate (un)compression step (using a temporary file) with the according facility, sufficient
              support provided.  Likewise, if the named file doesn't exist, but a file with one of the mentioned
              compression extensions does, then the name is automatically expanded and the compressed file is
              used.

              Otherwise, if the name ends with an extension for which file-hook-load-EXTENSION and
              file-hook-save-EXTENSION variables are set, then the given hooks will be used to load and save
              “name”, and S-nail will work with an intermediate temporary file.

              MBOX files (flat file-based mailboxes) are generally locked during file operations in order to
              avoid inconsistencies against concurrent modifications.  Mailbox files which S-nail treats as
              system mailboxes will also be protected by so-called dotlock files, the traditional way of mail
              spool file locking: for any file ‘a’ a lock file ‘a.lock’ will be created for the duration of the
              synchronization — as necessary a privilege-separated dotlock child process will be used to
              accommodate for necessary privilege adjustments in order to create the dotlock file in the same
              directory and with the same user and group identities as the file of interest.  Also see
              mbox-rfc4155 for fine-tuning the handling of MBOX files.

              If name refers to a directory with the subdirectories ‘tmp’, ‘new’ and ‘cur’, then it is treated
              as a folder in “Maildir” format.  A name of the form

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

              is taken as an Internet mailbox specification.  The [Option]ally supported protocols are pop3
              (POP3) and pop3s (POP3 with SSL/TLS encrypted transport).  The [/path] part is valid only for
              IMAP; there it defaults to INBOX.  Also see the section On URL syntax and credential lookup.

              [no v15-compat] If user contains special characters, in particular ‘/’ or ‘%’, they must be
              escaped in URL notation – the command urlencode can be used to show the necessary conversion.  The
              optional `path' part applies to IMAP only; if it is omitted, the default `INBOX' is used.

              If S-nail is connected to an IMAP server, a name of the form `@mailbox' refers to the `mailbox' on
              that server, but otherwise a `@' prefix has no special meaning.

     flag     Takes a message list and marks the messages as flagged for urgent/special attention.  This mark
              has no technical meaning in the mail system; it just causes messages to be highlighted in the
              header summary, and makes them specially addressable.

     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; e.g. the command `folders @'
              lists the folders on the base level of the current IMAP server.  See also the variable
              imap-list-depth.

     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  Alias for Fwd.

     forward  Alias for fwd.

     from     (f) Takes a list of messages and prints their message headers (which qualifies this command as
              S-nails search facility).

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

     fwd      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 fwdheading variable printed
              before.  The fwdignore and fwdretain commands specify which header fields are included in the new
              message.  Only the first part of a multipart message is included unless the forward-as-attachment
              option is set.  Unless the option fullnames is set recipient addresses will be stripped from
              comments, names etc.

     fwdignore
              Specifies which header fields are to be ignored with the command fwd.  This command has no effect
              when the forward-as-attachment option is set.

     fwdretain
              Specifies which header fields are to be retained with the command fwd.  fwdretain overrides
              fwdignore.  This command has no effect when the forward-as-attachment option is set.

     ghost    Define or list command aliases, so-called ghosts.  Without arguments a list of all currently known
              aliases is printed.  With one argument the expansion of the given alias is shown.  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.
              A ghost 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 content, and the resulting
              string forms the command line that is, in effect, executed.  Also see unghost.

                    ? gh xx
                    `ghost': no such alias: "xx"
                    ? gh xx echo hello,
                    ? gh xx
                    ghost xx "echo hello,"
                    ? xx
                    hello,
                    ? xx world
                    hello, world

     headers  (h) Show the current group of headers, the size of which depends on the variable screen.  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”.

     help     (hel) A synonym for ?.

     history  [Option] Either show or clear the list of history entries; a decimal NUMBER argument selects and
              shows the respective history entry – press “ENTER” to accept it, and the history entry will become
              the new history top.  The default mode if no arguments are given is show.

     hold     (ho, also preserve) Takes a message list and marks each message therein to be saved in the user's
              system mailbox instead of in 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 nestable if/elif/else/endif conditional execution construct — if the given
              condition is true then the encapsulated block is executed.  POSIX only supports the conditions
              ‘[Rr]eceive’, ‘[Ss]end’ and ‘[Tt]erm’ (execute if standard input is a tty), all remaining
              conditions are non-portable extensions; note that falsely specified conditions cause the execution
              of the entire conditional construct until the (matching) closing endif command to be suppressed.
              The syntax of the nestable if conditional execution construct requires that each condition and
              syntax element is surrounded by whitespace.

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

              Another condition can be any boolean value (see the section Value options for booleans) to mark an
              enwrapped block as “never execute” or “always execute”.  It is possible to check a variable for
              existence or compare its expansion against a user given value or another variable via the ‘$’
              (“variable next”) conditional trigger character.  The available comparison 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).  The values of the
              left and right hand side are treated as strings and are compared 8-bit byte-wise, ignoring case
              according to the rules of the US-ASCII encoding (therefore, dependend on the active locale,
              possibly producing false results for strings in the locale encoding).  Except for the substring
              checks the comparison will instead be performed arithmetically if both, the user given value as
              well as the variable content, can be parsed as numbers (integers).  An unset variable is treated
              as the empty string.

              When the [Option]al regular expression support is available, the additional test cases ‘=~’ and
              ‘!~’ can be used.  They treat the right hand side as a regular expression that is matched case-
              insensitively and according to the active LC_CTYPE locale, meaning that strings in the locale
              encoding should be matched 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.

                    if $debug
                      echo *debug* is set
                    endif
                    if $ttycharset == "UTF-8"
                      echo *ttycharset* is set to UTF-8, case-insensitively
                    endif
                    set t1=one t2=one
                    if $t1 == $t2
                      echo These two variables are equal
                    endif
                    if $version-major >= 15
                      echo Running a new version..
                      if $features =@ "regex"
                        if $TERM =~ "^xterm.*"
                          echo ..in an X terminal
                        endif
                      endif
                      if [ [ true ] && [ [ $debug ] || [ $verbose ] ] ]
                        echo Noisy, noisy
                      endif
                      if true && $debug || $verbose
                        echo Left associativity, as is known from the shell
                      endif
                      if ! ! true && ! [ ! $debug && ! $verbose ]
                        echo Unary operator support
                      endif
                    endif

     ignore   Without arguments the list of ignored header fields is printed, otherwise the given list of header
              fields is added to the ignore list: Header fields in the ignore list are not printed on the
              terminal when a message is printed.  To print a message in its entirety, use the commands Type or
              Print.  Also see discard and retain.

     imap     [Option] 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.

     inc      Same as newmail.

     list     Prints the names of all available commands, alphabetically sorted.

     localopts
              This command can be used to localize changes to variables, meaning that their state will be
              reverted to the former one once the covered scope is left.  It can only be used inside of macro
              definition blocks introduced by account or define, and is interpreted as a boolean (see Value
              options); the “covered scope” of an account is left once it is switched off again.

                    define temporary_settings {
                      set global_option1
                      localopts on
                      set local_option1
                      set local_option2
                      localopts off
                      set global_option2
                    }

              Note that 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 by ‘macro1’!

     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
              send 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.

     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.

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

     mimetype
              Without any arguments the content of the MIME type cache will displayed.  Otherwise each argument
              defines a complete MIME type specification of a type that shall 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.  Refer to the section on The mime.types files for
              more on MIME type specifications and this topic in general.  MIME type unregistration and cache
              resets can be triggered with unmimetype.

     mlist    Without arguments the list of all currently defined mailing lists (and their attributes, if any)
              is printed.  Otherwise all given arguments (which need not be quoted except for whitespace) will
              be added and henceforth be recognized as mailing lists.  Mailing lists may be removed via the
              command unmlist.

              If the [Option]al regular expression support is available then mailing lists may also be specified
              as regular expressions (see re_format(7) for more on those).

     mlsubscribe
              Without arguments the list of all currently defined mailing lists which have a subscription
              attribute is printed.  Otherwise this attribute will be set for all given mailing lists, newly
              creating them as necessary (as via mlist).  Subscription attributes may be removed via the command
              unmlsubscribe.  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 prints ignored header fields 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] Either (show or) clear the current .netrc cache, loading the file first as necessary in
              the former case.  Note that S-nail will try to read the file only once, use ‘netrc clear’ to
              unlock the next attempt.  See netrc-lookup 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 printed.  If the header variable is set, the headers of each new message are
              also printed.

     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 prints ignored header fields 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 ignored header fields and all parts of MIME ‘multipart/alternative’
              messages.

     pipe     (pi) Takes a message list and a shell command and pipes the messages through the command.  Without
              an argument the current message is piped through the command given by the cmd variable.  If the
              page variable is set, every message is followed by a formfeed character.

     preserve
              (pre) A synonym for hold.

     Print    (P) Like print but also prints out ignored header fields and all parts of MIME
              ‘multipart/alternative’ messages.  See also print, ignore and retain.

     print    (p) Takes a message list and types out each message on the user's terminal.  For MIME multipart
              messages, all parts with a content type of ‘text’ or ‘message’ are shown, the other are hidden
              except for their headers.  Messages are decrypted and converted to the terminal character set if
              necessary.

     quit     (q) Terminates the session, saving all undeleted, unsaved messages in the current MBOX, preserving
              all messages marked with hold or preserve or never referenced in the system mailbox, and removing
              all other messages from the 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 flag -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.

     redirect
              Same as resend.

     Redirect
              Same as Resend.

     remove   Removes the named folders.  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.  Both folders must be of the same type and must be located on the current server for
              IMAP.

     Reply    (R) Reply to originator.  Does not reply to other recipients of the original message.  flipr will
              exchange this command with reply.  Unless the option fullnames is set the recipient address will
              be stripped from comments, names etc.

     reply    (r) Take a message and group-responds to it by addressing the sender and all recipients.
              followup-to, followup-to-honour, reply-to-honour as well as recipients-in-cc influence response
              behaviour.  The command Lreply offers special support for replying to mailing lists.  Unless the
              option fullnames is set recipient addresses will be stripped from comments, names etc.  If flipr
              is set the commands Reply and reply are exchanged.

     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.

     Respond  Same as Reply.

     respond  Same as reply.

     respondall
              Same as replyall.

     respondsender
              Same as replysender.

     retain   (ret) Without arguments the list of retained header fields is printed, otherwise the given list of
              header fields is added to the retain list: Header fields in the retain list are shown on the
              terminal when a message is printed, all other header fields are suppressed.  To print a message in
              its entirety, use the commands Type or Print.  Also see discard and ignore; retain takes
              precedence over the mentioned.

     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.

     save     (s) Takes a message list and a filename and appends each message in turn to the end of the file.
              If no filename is given, the MBOX file is used.  The filename in quotes, followed by the generated
              character count is echoed on the user's terminal.  If editing a system mailbox the messages are
              marked for deletion.  Compressed files and IMAP mailboxes are handled as described for the -f
              command line option above.

     savediscard
              Same as saveignore.

     saveignore
              Is to save what ignore is to print and type.  Header fields thus marked are filtered out when
              saving a message by save or when automatically saving to MBOX.  This command should only be
              applied to header fields that do not contain information needed to decode the message, as MIME
              content fields do.  If saving messages on an IMAP account ignoring fields makes it impossible to
              copy the data directly on the server, thus operation usually becomes much slower.

     saveretain
              Is to save what retain is to print and type.  Header fields thus marked are the only ones saved
              with a message when saving by save or when automatically saving to MBOX.  saveretain overrides
              saveignore.  The use of this command is strongly discouraged since it may strip header fields that
              are needed to decode the message correctly.

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

     set      (se) Without arguments this command prints all options and, for non-binary options, values that
              are currently known to S-nail.  Setting any of the options bsdcompat or bsdset changes the output
              format to BSD style, otherwise a properly quoted listing is produced.  If debug is set or verbose
              has been set twice then the listing is modified to mark out assembled variables.

              Otherwise modifies (set and unsets) the given options.  Arguments are of the form ‘option=value’
              (no space before or after ‘=’), or plain ‘option’ if there is no value.  Quotation marks may be
              placed around any part of the assignment statement to quote blanks or tabs, e.g.,

                    set indentprefix="->"

              If an argument 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’).

     setenv   Identical to set except that the options are also exported into the program environment; since
              this task requires native host support the command will always report error if that is not
              available (but still act like set in this case).  This operation is a no-op unless all resource
              files have been loaded.  Also see unsetenv.

     shell    (sh) Invokes an interactive version of the shell.

     shortcut
              Without arguments the list of all currently defined shortcuts is printed.  Otherwise all given
              arguments (which need not be quoted except for whitespace) are treated as pairs of shortcuts and
              their expansions, creating new or changing already existing shortcuts, as necessary.  Shortcuts
              may be removed via the command unshortcut.  The expansion strings should be in the syntax that has
              been described for the file command.

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

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

     sort     Create a sorted representation of the current folder, and change 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
              printed.  Possible sorting criteria 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.

              If no argument is given, the current sorting criterion is printed.

     source   (so) The source command reads commands from a file.

     source_if
              The difference to source is that this command will not generate an error if the given file
              argument cannot be opened successfully.  This can matter in, e.g., resource files, since loading
              of those is stopped when an error is encountered.

     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 wether 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 ‘ghost’ as necessary).  Create a threaded
              representation of the current folder, i.e. indent messages that are replies to other messages in
              the header display and change the next command and the addressing modes such that they refer to
              messages in the threaded order.  Message numbers are the same as in unthreaded mode.  If the
              header variable is set, a header summary in threaded order is also printed.

     top      (to) Takes a message list and prints the top few lines of each.  The number of lines printed is
              controlled by the variable toplines and defaults to five.

     touch    (tou) Takes a message list and marks the messages for saving in 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) Identical to the Print command.

     type     (t) A synonym for print.

     unaccount
              Delete all given accounts.  An error message is printed if a given account is not defined.  The
              special name ‘*’ will discard all existing accounts.

     unalias  (una) Takes a list of names defined by alias commands and discards the remembered groups of users.
              The special name ‘*’ will discard all existing aliases.

     unanswered
              Takes a message list and marks each message as not having been answered.

     uncollapse
              Only applicable to threaded mode.  Takes a message list and makes the message and all replies to
              it visible in header summaries again.  When a message becomes the current message, it is
              automatically made visible.  Also when a message with collapsed replies is printed, all of these
              are automatically uncollapsed.

     undefine
              Undefine all given macros.  An error message is printed if a given macro is not defined.  The
              special name ‘*’ will discard all existing macros.

     undelete
              (u) Takes a message list and marks each message as not being deleted.

     undraft  Takes a message list and undrafts each message.

     unflag   Takes a message list and marks each message as not being flagged.

     unfwdignore
              Removes the header field names from the list of ignored fields for the forward command.  The
              special name ‘*’ will remove all fields.

     unfwdretain
              Removes the header field names from the list of retained fields for the forward command.  The
              special name ‘*’ will remove all fields.

     unghost  Remove all the given command ghosts.  The special name ‘*’ will remove all ghosts.

     unignore
              Removes the header field names from the list of ignored fields.  The special name ‘*’ will remove
              all fields.

     unmimetype
              Delete all given MIME types, e.g., ‘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.

     unmlist  Forget about all the given mailing lists.  The special name ‘*’ will remove all lists.  Also see
              mlist.

     unmlsubscribe
              Remove the subscription attribute from all given mailing lists.  The special name ‘*’ will clear
              the attribute from all lists which have it set.  Also see mlsubscribe.

     Unread   Same as unread.

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

     unretain
              Removes the header field names from the list of retained fields.  The special name ‘*’ will remove
              all fields.

     unsaveignore
              Removes the header field names from the list of ignored fields for saving.  The special name ‘*’
              will remove all fields.

     unsaveretain
              Removes the header field names from the list of retained fields for saving.  The special name ‘*’
              will remove all fields.

     unset    (uns) Takes a list of option names and discards their remembered values; the inverse of set.

     unsetenv
              Identical to unset except that the options are also removed from the program environment; since
              this task requires native host support the command will always report error if that is not
              available (but still act like unset).  This operation is a no-op unless all resource files have
              been loaded.  Also see setenv.

     unshortcut
              Deletes the shortcut names given as arguments.  The special name ‘*’ will remove all shortcuts.

     unsort   Disable sorted or threaded mode (see the sort and thread commands), return to normal message order
              and, if the header variable is set, print a header summary.

     unthread
              [Obsolete] Same as unsort.

     urldecode
              Decode the given URL-encoded string arguments and show the results.

     urlencode
              URL-encode the given arguments and show the results.

     varedit  Edit the values of or create the given variable(s) in the EDITOR.  Binary variables cannot be
              edited.

     varshow  Show informations about all the given options.  S-nail knows about a finite set of known builtin
              variables that are subdivided further in binary and value variants; they may have special
              properties, like “read-only” (setting may not be changed) and “virtual”, meaning that the value is
              generated on-the-fly as necessary.  Beside those known variables an infinite number of unknown,
              so-called “assembled” variables, which are expected to be able to store values, may exist.

                    ? set foo=bar nobar
                    ? varshow sendwait version-major foo bar
                    "sendwait": (73) binary: set=1 (ENVIRON=0)
                    "version-major": (192) value, read-only, virtual:\
                      set=1 (ENVIRON=0) value<14>
                    "foo": (assembled) set=1 (ENVIRON=0) value<bar>
                    "bar": (assembled) set=0 (ENVIRON=0) value<NULL>

     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.

     visual   (v) Takes a message list and invokes the display editor on each message.  Modified contents are
              discarded unless the writebackedited variable is set.

     write    (w) For conventional messages the body without all headers is written.  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, and the user is asked for a filename to save each other part.  For
              convience saving of each part may be skipped by giving an empty value; the same result can also be
              achieved by writing it to /dev/null.  For the second and subsequent parts a leading ‘|’ character
              causes the part to be piped to the remainder of the user input interpreted as a shell command;
              otherwise the user input is expanded as usually for folders, e.g., tilde expansion is performed.
              In non-interactive mode, only the parts of the multipart message that have a filename given in the
              part header are written, the others are discarded.  The original message is never marked for
              deletion in the originating mail folder.  For attachments, the contents of the destination file
              are overwritten if the file previously existed.  No special handling of compressed files is
              performed.

     xit      (x) A synonym for exit.

     z        S-nail presents message headers in windowfuls as described under the headers command.  This
              command scrolls to the next window of messages.  If an argument is given, it specifies the window
              to use.  A number prefixed by ‘+’ or ‘-’ indicates that the window is calculated in relation to
              the current position.  A number without a prefix specifies an absolute window number, and a ‘$’
              lets S-nail scroll to the last window of messages.

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

TILDE ESCAPES

     Here is a summary of the tilde escapes, which are used to perform special functions when composing
     messages.  Tilde escapes are only recognized at the beginning of lines.  The name “tilde escape” is
     somewhat of a misnomer since the actual escape character can be changed by adjusting the option escape.

     ~~ 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 in order to send it at the beginning of a
                  line.)

     ~! command   Execute the indicated shell command, then return to the message.

     ~.           Same effect as typing the end-of-file character.

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

     ~?           Write a summary of command escapes.

     ~< filename  Identical to ~r.

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

     ~@ [filename...]
                  With no arguments, edit the attachment list interactively.  If an attachment's file name is
                  left empty, that attachment is deleted from the list.  When the end of the attachment list is
                  reached, S-nail will ask for further attachments until an empty name is given.  If a given
                  file name solely consists of the number sign ‘#’ followed by a valid message number of the
                  currently active mailbox, then the given message is attached as a MIME ‘message/rfc822’ and
                  the rest of this section does not apply.

                  If character set conversion has been compiled into S-nail, then this mode gives the user the
                  option to specify input and output character sets, unless the file extension indicates binary
                  content, in which case S-nail asks wether this step shall be skipped for the attachment in
                  question.  If not skipped, then the charset that succeeds to represent the attachment data
                  will be used in the ‘charset=’ MIME parameter of the mail message:

                     If input and output character sets are specified, then the conversion is performed on the
                      fly.  The user will be asked repeatedly until the desired conversion succeeds.
                     If only an output character set is specified, then the input is assumed to be in the
                      ttycharset charset and will be converted to the given output charset on the fly.  The user
                      will be asked repeatedly until the desired conversion succeeds.
                     If no character sets are specified at all then the algorithm that is documented in the
                      section Character sets is applied, but directly and on the fly.  The user will be asked
                      repeatedly until the desired conversion succeeds.
                     Finally, if an input-, but no output character set is specified, then no conversion is
                      ever performed, but the ‘charset=’ MIME parameter value will still be set to the user
                      input.
                     The character set selection loop can be left by typing ‘control-C’, i.e., causing an
                      interrupt.  Note that before S-nail version 15.0 this terminates the entire current
                      attachment selection, not only the character set selection.

                  Without character set conversion support, S-nail will ask for the input character set only,
                  and it'll set the ‘charset=’ MIME parameter value to the given input, if any; if no user input
                  is seen then the ttycharset character set will be used for the parameter value instead.  Note
                  that the file extension check isn't performed in this mode, since no conversion will take
                  place anyway.

                  Note that in non-interactive mode, for reproduceabilities sake, there will always be two
                  questions for each attachment, regardless of wether character set conversion is available and
                  what the file extension is.  The first asks for the filename, and the second asks for the
                  input character set to be passed through to the corresponding MIME parameter; no conversion
                  will be tried if there is input to the latter question, otherwise the usual conversion
                  algorithm, as above, is applied.  For message attachments, the answer to the second question
                  is completely ignored.

                  If (instead) filename arguments are specified for the ~@ command they are treated as a comma
                  separated list of files, which are all expanded and appended to the end of the attachment
                  list.  (Filenames with commas, or with leading or trailing whitespace can only be added via
                  the command line or the first method.  Message attachments can only be added via the first
                  method; filenames which clash with message numbers can only be added via the command line or
                  the second method.)  In this mode the (text) attachments are assumed to be in ttycharset
                  encoding, and will be evaluated as documented in the section Character sets.

     ~A           Inserts the string contained in the Sign variable (same as ‘~i Sign’).  The escape sequences
                  tabulator ‘\t’ and newline ‘\n’ are understood.

     ~a           Inserts the string contained in the sign variable (same as ‘~i sign’).  The escape sequences
                  tabulator ‘\t’ and newline ‘\n’ are understood.

     ~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.  After the editing session is
                  finished, the user may continue appending text to the message.

     ~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.

     ~f messages  Read the named messages into the message being sent.  If no messages are specified, read in
                  the current message.  ignore and retain lists are used to modify the message headers.  For
                  MIME multipart messages, only the first printable part is included.

     ~H           Edit the message header fields ‘From:’, ‘Reply-To:’, ‘Sender:’ and ‘Organization:’ by typing
                  each one in turn and allowing the user to edit the field.  The default values for these fields
                  originate from the from, replyto, sender and ORGANIZATION 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, adding a newline character at the
                  end.  The message remains unaltered if the variable is unset or empty.  The escape sequences
                  tabulator ‘\t’ and newline ‘\n’ are understood.

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

     ~m messages  Read the named messages into the message being sent, indented by indentprefix.  If no messages
                  are specified, read the current message.  ignore and retain lists are used to modify the
                  message headers.  For MIME multipart messages, only the first printable part is included.

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

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

     ~R filename  Read the named file into the message, indented by indentprefix.

     ~r filename  Read the named file into the message.

     ~s string    Cause the named string to become the current subject field.

     ~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 an alternate editor (defined by the VISUAL option) on the message collected so far.
                  Usually, the alternate editor will be a screen editor.  After the editor is quit, the user may
                  resume appending text to the end of the message.

     ~w filename  Write the message onto the named file.  If the file exists, the message is appended to it.

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

     ~| command   Pipe the message 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.

VARIABLE OPTIONS

     Variables are controlled via set and unset commands; in general using unset can also be accomplished by
     prefixing a variable name with the string “no” and calling set, e.g., ‘unset crt’ will have the same effect
     as ‘set nocrt’.  Creation or editing of variables in an editor can also be achieved with varedit.  varshow
     will give more insight on the given variable(s), whereas set will print a listing of all variables when
     called without arguments.  Options are also implicitly inherited from the program ENVIRONMENT and can be
     set explicitly via the command line option -S.

     Different kind of options exist: binary options, which can only be in one of the two states “set” and
     “unset”, as well as value options which have an assigned string value, for which proper quoting may be
     important upon assignment time.

   Initial Settings
     The standard POSIX 2008/Cor 1-2013 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 ‘? ’
     (note that S-nail deviates from the standard by using ‘\& ’, but the ‘\&’ special prompt escape results in
     “?” being printed unless bsdcompat is set), noquiet, norecord, save, nosendwait, noshowto, noSign, nosign,
     toplines set to ‘5’.

     Notes: S-nail doesn't support the noonehop variable – use command line options or sendmail-arguments to
     pass options through to a MTA.  And the default global s-nail.rc file (which is loaded unless the -n
     command line flag has been used or the NAIL_NO_SYSTEM_RC environment variable is set) bends those initial
     settings a bit, e.g., it sets the options hold, keepsave and keep, to name a few, calls retain etc., and
     should thus be taken into account.

   Binary options
     add-file-recipients
                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     Causes only the local part to be evaluated when comparing addresses.

     append     Causes messages saved in MBOX to be appended to the end rather than prepended.  This should
                always be set.

     ask or asksub
                Causes S-nail to prompt for the subject of each message sent.  If the user responds with simply
                a newline, no subject field will be sent.

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

     askattach  If set, S-nail asks for files to attach at the end of each message, shall the list be found
                empty at that time.  An empty line finalizes the list.

     askcc      Causes the user to be prompted for carbon copy recipients (at the end of each message if
                askatend or bsdcompat are set) shall the list be found empty (at that time).  An empty line
                finalizes the list.

     askbcc     Causes the user to be prompted for blind carbon copy recipients (at the end of each message if
                askatend or bsdcompat are set) shall the list be found empty (at that time).  An empty line
                finalizes the list.

     asksign    [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.

     autocollapse
                Causes threads to be collapsed automatically when threaded mode is entered (see the collapse
                command).

     autoprint  Causes the delete command to behave like ‘dp -’; thus, after deleting a message the next one
                will be typed automatically.

     autothread
                [Obsolete] Causes threaded mode (see the thread command) to be entered automatically when a
                folder is opened.  The same as ‘autosort=thread’.

     bang       Enables the substitution of ‘!’ by the contents of the last command line in shell escapes.

     batch-exit-on-error
                If the batch mode has been enabled via the -# command line option, then this variable will be
                consulted whenever S-nail completes one operation (returns to the command prompt); if it is set
                then S-nail will terminate if the last operation generated an error.

     bsdannounce
                Causes automatic display of a header summary after executing a file command.

     bsdcompat  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 meaning of the S-nail specific
                ‘\&’ prompt escape sequence.

     bsdflags   Changes the letters printed in the first column of a header summary to traditional BSD style.

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

     bsdmsgs    Changes some informational messages to traditional BSD style.

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

     bsdset     Changes the output format of the set command to traditional BSD style.

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

     colour-pager
                [Option] Wether colour shall be used for output that is paged through PAGER.  Note that pagers
                may need special flags, e.g., less(1) requires the option -R and lv(1) the option -c in order to
                support colours; therefore S-nail will inspect the variable PAGER – if that starts with the
                string “less” a non-existing environment variable LESS will be set to ‘FRSXi’, likewise for “lv”
                LV will optionally be set to “-c”.  Also see the section Coloured message display for more on
                this.

     debug      Prints debugging messages and disables the actual delivery of messages.  Also implies norecord
                and nosave.

     disconnected
                [Option] 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.

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

     dot        When dot is set, a dot ‘.’ on a line by itself during message input from a terminal shall be
                treated as end-of-message (in addition to the normal end-of-file condition).  If ignoreeof is
                set nodot is ignored and using a dot is the only method to terminate input mode.

     dotlock-ignore-error
                [Option] Synchronization of mailboxes which S-nail treats as system mailboxes (see the command
                file) will be protected with so-called dotlock files—the traditional mail spool file locking
                method—in addition to system file locking.  Because S-nail ships with a privilege-separated
                dotlock creation program that should always be able to create such a dotlock file there is no
                good reason to ignore dotlock file creation errors, and thus these are fatal unless this
                variable is set.

     editalong  If this variable is set then the editor is started automatically when a message is composed in
                interactive mode, as if the ~e TILDE ESCAPES had been specified.  The editheaders variable is
                implied for this automatically spawned editor session.

     editheaders
                When a message is edited while being composed, its header is included in the editable text.  The
                ‘To:’, ‘Cc:’, ‘Bcc:’, ‘Subject:’, ‘From:’, ‘Reply-To:’, ‘Sender:’, and ‘Organization:’ fields
                are accepted within the header, other fields are ignored.

     emptystart
                If the mailbox is empty S-nail normally prints “No mail for user” and exits immediately.  If
                this option is set S-nail starts even with an empty mailbox.

     flipr      This option 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.

     followup-to
                Controls wether 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.

     forward-as-attachment
                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 option messages are sent as unmodified MIME
                ‘message/rfc822’ attachments with all of their parts included.

     fullnames  When replying to or forwarding a message S-nail normally removes the comment and name parts of
                email addresses.  If this variable is set such stripping is not performed, and comments, names
                etc. are retained.

     header     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; enabled by default.  The command line
                option -N can be used to set noheader.

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

     history-gabby-persist
                [Option] S-nails own NCL will not save the additional (gabby) history entries in persistent
                storage unless this variable is also set.  Also see NAIL_HISTFILE.

     hold       This option is used to hold messages in the system mailbox by default.

     idna-disable
                [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).

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

     ignoreeof  Ignore end-of-file conditions (‘control-D’), on message input, which instead can be terminated
                only by entering a dot ‘.’ on a line by itself or by using the ~. TILDE ESCAPES.  This option
                also applies to S-nail command mode.

     imap-use-starttls-USER@HOST, imap-use-starttls-HOST, imap-use-starttls
                [Option] Causes S-nail to issue a `STARTTLS' command to make an unencrypted IMAP session SSL/TLS
                encrypted.  This functionality is not supported by all servers, and is not used if the session
                is already encrypted by the IMAPS method.

     keep       If set, an empty mailbox file is not removed.  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.  Note this only applies to local
                regular (MBOX) files, other mailbox types will never be removed.

     keepsave   When a message is saved it is usually discarded from the originating folder when S-nail is quit.
                Setting this option causes all saved message to be retained.

     line-editor-disable
                Turn off any enhanced command line editing capabilities (see Command line editor for more).

     markanswered
                When a message is replied to and this variable is set, it is marked as having been answered.
                This mark has no technical meaning in the mail system; it just causes messages to be marked in
                the header summary, and makes them specially addressable.

     mbox-rfc4155
                S-nail produces and expects fully RFC 4155 compliant MBOX text mailboxes.  Messages which are
                fetched over the network or from within already existing Maildir (or any non-MBOX) mailboxes may
                require so-called ‘From_’ quoting (insertion of additional ‘>’ characters to prevent line
                content misinterpretation) to be applied in order to be storable in MBOX mailboxes, however,
                dependent on the circumspection of the message producer.  (E.g., S-nail itself will, when newly
                generating messages, choose a Content-Transfer-encoding that prevents the necessity for such
                quoting – a necessary precondition to ensure message checksums won't change.)

                By default S-nail will perform this ‘From_’ quoting in a way that results in a MBOX file that is
                compatible with the loose POSIX MBOX layout, in order not to exceed the capabilities of simple
                applications, however.  Set this option to generate MBOX files for RFC 4155 compliant
                applications only.

     message-id-disable
                By setting this option the generation of ‘Message-ID:’ can be completely suppressed, effectively
                leaving this task up to the mail-transfer-agent (MTA) or the SMTP server.  (According to RFC
                5321 your SMTP server is not required to add this field by itself, so you should ensure that it
                accepts messages without a ‘Message-ID’.)

     metoo      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 to mail-transfer-agents (MTAs); though most of the modern MTAs don't (no longer)
                document this flag, no MTA is known which doesn't support it (for historical compatibility).

     mime-allow-text-controls
                When sending messages, each part of the message is MIME-inspected in order to classify the
                ‘Content-Type:’ and ‘Content-Transfer-Encoding:’ (see 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 option 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.

     netrc-lookup-USER@HOST, netrc-lookup-HOST, netrc-lookup
                [v15-compat] [Option] Used to control usage of the users .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.

     outfolder  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 to the current directory, unless it is set to an absolute pathname.

     page       If set, each message the pipe command prints out is followed by a formfeed character ‘\f’.

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

     pop3-bulk-load-USER@HOST, pop3-bulk-load-HOST, pop3-bulk-load
                [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 option is set then S-nail will download only complete
                messages from the given POP3 server(s) instead.

     pop3-no-apop-USER@HOST, pop3-no-apop-HOST, pop3-no-apop
                [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
                [Option] Causes S-nail to issue a ‘STLS’ command to make an unencrypted POP3 session SSL/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].

     print-all-chars
                This option causes all characters to be considered printable.  It is only effective if given in
                a startup file.  With this option set some character sequences in messages may put the user's
                terminal in an undefined state when printed; it should only be used as a last resort if no
                working system locale can be found.

     print-alternatives
                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’.

     quiet      Suppresses the printing of the version when first invoked.

     quote-as-attachment
                If this is set, then the original message is added in its entirety as a ‘message/rfc822’ MIME
                attachment when replying to a message.  Note this works regardless of the setting of quote.

     recipients-in-cc
                On group replies, specify only the sender of the original mail in ‘To:’ and mention the other
                recipients in the secondary ‘Cc:’.  By default all recipients of the original mail will be
                addressed via ‘To:’.

     record-resent
                If both this variable and the record variable are set, the resend and Resend commands save
                messages to the record folder as it is normally only done for newly composed messages.

     reply-in-same-charset
                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.

     rfc822-body-from_
                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       Enable saving of (partial) messages in DEAD upon interrupt or delivery error.

     searchheaders
                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-else-ttycharset
                [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 (given that ttycharset hasn't been
                set manually), i.e., without converting it to the charset-8bit fallback character set.  Thus,
                mail message text will 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.  If no character set conversion
                capabilities are available in S-nail then the only supported character set is ttycharset.

     sendmail-no-default-arguments
                Unless this option is set S-nail will pass some well known standard command line options to the
                defined sendmail program, see there for more.

     sendwait   When sending a message wait until the MTA (including the builtin 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   Setting this option causes S-nail to start at the last message instead of the first one when
                opening a mail folder.

     showname   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     Causes the recipient of the message to be shown in the header summary if the message was sent by
                the user.

     skipemptybody
                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-force-encryption
                [Option] Causes S-nail to refuse sending unencrypted messages.

     smime-sign
                [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-message-digest.

     smime-no-default-ca
                [Option] Don't load default CA locations when verifying S/MIME signed messages.

     smtp-use-starttls-USER@HOST, smtp-use-starttls-HOST, smtp-use-starttls
                [Option] Causes S-nail to issue a ‘STARTTLS’ command to make an SMTP session SSL/TLS encrypted,
                i.e., to enable transport layer security.

     ssl-no-default-ca
                [Option] Don't load default CA locations to verify SSL/TLS server certificates.

     term-ca-mode
                [Option] If terminal capability queries are supported and this option is set then S-nail will
                try to switch to the “alternate screen” when in interactive mode, so that the terminal will go
                back to the normal screen, leaving all the text there intact, when S-nail exits.  Note: even
                when supported for this to produce appealing results the used PAGER and possibly configured
                pipe-TYPE/SUBTYPE applications that take control over the terminal need to have corresponding
                support too, e.g., the less(1) pager should be driven with the ‘-X’ command line flag.

     keep-content-length
                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-compat
                Setting this option 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    Setting this option, also controllable via the command line option -v, causes S-nail to be more
                verbose, so that, e.g., certificate chains will be displayed on the users terminal.  Setting
                this binary option twice increases the level of verbosity, 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.

     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 excercise to the user.

   Value options
     Options with values that are generally treated as strings.  To embed whitespace (space and tabulator) in a
     value it either needs to be escaped with a backslash character, or the entire value must be enclosed in
     (double or single) quotation marks; To use quotation marks identical to those used to enclose the value,
     escape them with a backslash character.  The backslash character has no special meaning except in these
     cases.

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

     Booleans are special string values that must either be set to decimal integers (in which case ‘0’ is false
     and ‘1’ and any other value is true) or to any of ‘off’, ‘no’ and ‘false’ for a false boolean and ‘on’,
     ‘yes’ and ‘true’ for a true boolean; matching is performed case-insensitively.  And there exists a special
     kind of boolean, the “quadoption”: this is expected to either name a boolean or one of the strings
     ‘ask-yes’ and ‘ask-no’ (in fact: ‘ask-’ followed by a valid boolean, case-insensitively); if one of the
     latter is set then in interactive mode the user will be prompted with the default value (also used for
     empty user input) set to the given boolean, whereas in non-interactive the given default will be used right
     away.

     agent-shell-lookup-USER@HOST, agent-shell-lookup-HOST, agent-shell-lookup
                [v15-compat] [Option] Account passwords can be fetched via an external agent program in order to
                permit encrypted password storage – see On URL syntax and credential lookup for more on
                credential lookup.  If this is set then the content is interpreted as a shell command the output
                of which (with newline characters removed) is treated as the account password shall the command
                succeed (and have produced non-empty non-newline output); e.g., via gpg(1):

                      $ echo PASSWORD > .pass
                      $ gpg -e .pass
                      $ eval `gpg-agent --daemon \
                          --pinentry-program=/usr/bin/pinentry-curses \
                          --max-cache-ttl 99999 --default-cache-ttl 99999`
                      $ echo 'set agent-shell-lookup="gpg -d .pass.gpg"' \
                          >> ~/.mailrc

                A couple of environment variables will be set for the agent:

                NAIL_TMPDIR       The temporary directory that S-nail uses.  Usually identical to TMPDIR, but
                                  guaranteed to be set and usable by child processes; to ensure the latter
                                  condition for TMPDIR also, it'll be set.
                NAIL_USER         The user (‘USER’) for which the password is looked up.
                NAIL_USER_ENC     The URL percent-encoded variant of NAIL_USER.
                NAIL_HOST         The plain machine hostname of the user account.
                NAIL_HOST_PORT    The ‘HOST’ (hostname possibly including port) of the user account.

     attrlist   A sequence of characters to print in the ‘attribute’ column of the headline as shown in the
                header display; each for one type of messages (see Message states), with the default being
                ‘NUROSPMFAT+-$~’ or ‘NU  *HMFAT+-$~’ if bsdflags or the SYSV3 environment variable are 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.
                      ‘+’       start of a collapsed thread.
                      ‘-’       collapsed.
                      ‘$’       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.

     autosort   Causes sorted mode (see the sort command) to be entered automatically with the value of this
                option as sorting method when a folder is opened.

     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 no character set conversion capabilities are
                available in S-nail then the only supported character set is ttycharset.  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.

     cmd        The default value for the pipe command.

     colour-from_
                [Option] The colour specification for so-called ‘From_’ lines.  See the section Coloured message
                display for the format of the value.

     colour-header
                [Option] The colour specification for header lines.

     colour-msginfo
                [Option] The colour specification for the introductional message info line.

     colour-partinfo
                [Option] The colour specification for MIME part info lines.

     colour-terms
                [Option] A comma-separated list of TERMinals for which coloured message display can be used.
                Entries only need to be added if the string “color” isn't part of the terminal name itself; the
                default value is

                      cons25,linux,rxvt,rxvt-unicode,screen,sun,vt100,vt220,wsvt25,xterm

     colour-uheader
                [Option] The colour specification for those header lines that have been placed in the
                colour-user-headers list.  See the section Coloured message display.

     colour-user-headers
                A comma separated list of (case-insensitive) header names which should be colourized with the
                alternative colour-uheader colours.  The default value is ‘from,subject’.

     crt        In a(n interactive) terminal session, then if this valued option is set it'll 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
                treshold (see LINES and stty(1)).

     datefield  The date in a header summary is normally the date of the mailbox ‘From_’ line of the message.
                If this variable is set, then the date as given in the ‘Date:’ field is used, converted to local
                time.  It is possible to control the display of the date by assigning a value, in which case the
                strftime(3) function will be used to format the date accordingly.  Please read your system
                manual for the available formats.  Note that the ‘%n’ format should not be used, because S-nail
                doesn't take embedded newlines into account when calculating how many lines fit onto the screen.

     datefield-markout-older
                This option, when set in addition to datefield, is used to display “older” messages (concept is
                rather comparable to the -l option of the POSIX utility ls(1)).  The content interpretation is
                identical to datefield.

     encoding   Suggestion for the MIME encoding to use in outgoing text messages and message parts.  Valid
                values are the default ‘quoted-printable’, ‘8bit’ and ‘base64’.  ‘8bit’ may cause problems when
                transferring mail messages over channels that are not ESMTP (RFC 1869) compliant.  If there is
                no need to encode a message, ‘7bit’ transfer mode is always used regardless of this variable.
                Binary data is always encoded as ‘base64’.

     escape     If defined, the first character of this option gives the character to use in place of ‘~’ to
                denote TILDE ESCAPES.

     expandaddr
                If not set then file and command pipeline targets are not allowed, and any such address will be
                filtered out, giving a warning message.  If set without a value then all possible recipient
                address specifications will be accepted – see the section Sending mail for more on this.  To
                accept them, but only in interactive mode, or when tilde commands were enabled explicitly by
                using one of the command line options -~ or -#, set this to the (case-insensitive) value
                ‘restrict’ (note right now this is actually like setting ‘restrict,-all,+name,+addr’).

                In fact the value is interpreted as a comma-separated list of values.  If it contains ‘fail’
                then the existence of disallowed specifications is treated as a hard send error instead of only
                filtering them out.  The remaining values specify wether a specific type of recipient address
                specification is allowed (optionally indicated by a plus sign ‘+’ prefix) or disallowed
                (prefixed with a hyphen ‘-’).  The value ‘all’ addresses all possible address specifications,
                ‘file’ file targets, ‘pipe’ command pipeline targets, ‘name’ plain user names and (MTA) aliases
                ([Obsolete] ‘noalias’ may be used as an alternative syntax to ‘-name’) and ‘addr’ network
                addresses.  These kind of values 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.

     expandargv
                Unless this variable is set additional mail-transfer-agent (MTA) arguments from the command
                line, as can be given after a ‘--’ separator, are ignored due to safety reasons.  However, if
                set to the special value ‘fail’, then the presence of additional MTA arguments is treated as a
                hard error that causes S-nail to exit with failure status.  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 -#.

     features   (Read-only) Information on the features compiled into S-nail – the content of this variable is
                identical to the output of the command features.

     file-hook-load-EXTENSION, file-hook-save-EXTENSION
                It is possible to install file hooks which will be used by the file command in order to be able
                to transparently handle (through an intermediate temporary file) files with specific
                ‘EXTENSION’s: the variable values can include shell snippets and are expected to write data to
                standard output / read data from standard input, respectively.  [v15 behaviour may differ] The
                variables may not be changed while there is a mailbox attendant.

                      set file-hook-load-xy='echo >&2 XY-LOAD; gzip -cd' \
                          file-hook-save-xy='echo >&2 XY-SAVE; gzip -c' \
                          record=+null-sent.xy

     folder     The name of the directory to use for storing folders of messages.  All folder names that begin
                with ‘+’ refer to files below it.  The same special conventions as documented for the file
                command may be used when specifying a new value for folder, but be aware that the expansion is
                fully performed immediately.  E.g., if the expanded name refers to an IMAP account, all names
                that begin with `+' refer to IMAP mailboxes below the folder target box.

                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).  Folder names prefixed
                by `+' will then refer to folders below `INBOX', while folder names prefixed by `@' refer to
                folders below the hierarchy base.  See the imap namespace command for a method to detect the
                appropriate prefix and delimiter.

     folder-hook
                When a folder is opened and this variable is set, the macro corresponding to the value of this
                variable is executed.  The macro is also invoked when new mail arrives, but message lists for
                commands executed from the macro only include newly arrived messages then.  If localopts are
                activated in a folder hook, then the covered settings will be reverted once the folder is left
                again.

     folder-hook-FOLDER
                Overrides folder-hook for a folder named ‘FOLDER’.  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’.

     followup-to-honour
                Controls wether 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.

     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.  If replying to messages these addresses
                are handled as if they were in the alternates list.  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] and with smtp smtp-hostname adds even more fine-tuning capabilities), have to be
                set.  If from contains more than one address, setting the sender variable is required (according
                to the standard RFC 5322).

     fwdheading
                The string to print before the text of a message with the forward command (unless the
                forward-as-attachment variable is set).  Defaults to “-------- Original Message --------” if
                unset; No heading is printed if it is set to the empty string.

     headline   A format string to use for the header summary, similar to printf(3) formats.  A percent
                character ‘%’ introduces a format specifier that may be followed by a number indicating the
                field width; If the (possibly implicitly implied) field width is negative, the field is to be
                left-aligned.  Valid format specifiers are:

                      ‘%%’  A plain percent character.
                      ‘%>’  A space character but for the current message, for which it expands to ‘>’.
                      ‘%<’  A space character but for the current message, for which it expands to ‘<’.
                      ‘%$’  [Option] The spam score of the message, as has been classified via the command
                            spamrate.  Prints 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 when the message was received.
                      ‘%e’  The indenting level in threaded mode.
                      ‘%f’  The address of the message sender.
                      ‘%i’  The message thread structure.  (Note that this format doesn't support a field
                            width.)
                      ‘%l’  The number of lines of the message.
                      ‘%m’  Message number.
                      ‘%o’  The number of octets (bytes) in the message.
                      ‘%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.

                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 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 printing 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.

     hostname   Use this string as hostname when expanding local addresses instead of the value obtained from
                uname(3) and getaddrinfo(3), i.e., in ‘Message-ID:’ and ‘From:’ fields.  Note that when smtp
                transport is not used then it is normally the responsibility of the MTA to create these fields,
                [v15-compat] in conjunction with smtp however smtp-hostname also influences the results; you
                should produce some test messages with the desired combination of hostname, and/or from, sender
                etc. first.

     imap-auth-USER@HOST, imap-auth
                [Option] 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
                [Option] 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-keepalive-USER@HOST, imap-keepalive-HOST, imap-keepalive
                [Option] 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
                [Option] 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.

     indentprefix
                String used by the ~m, ~M and ~R TILDE ESCAPES and by the quote option for indenting messages,
                in place of the normal tabulator character ‘^I’, which is the default.  Be sure to quote the
                value if it contains spaces or tabs.

     line-editor-cursor-right
                [Option] If the builtin command line editor is used, actions which are based on rightwise
                movement may not work on some terminals.  If you encounter such problems, set this variable to
                the terminal control sequence that is necessary to move the cursor one column to the right.  The
                default is ‘\033[C’, which should work for most terminals.  Less often occur ‘\033OC’ and
                ‘\014’.  Note that “Escape” and other control character have to be written as shell-style escape
                sequences, e.g., ‘\033’ for (US-ASCII) “Escape”.

     MAIL       Is used as the user's mailbox, if set.  Otherwise, a system-dependent default is used.  Supports
                a logical subset of the special conventions that are documented for the file command and the
                folder option.

     mime-counter-evidence
                Normally the ‘Content-Type:’ field is used to decide how to handle MIME parts.  Some MUAs
                however don't use mime.types(5) or a similar mechanism to correctly classify content, but simply
                specify ‘application/octet-stream’, even for plain text attachments like ‘text/diff’.  If this
                variable is set then S-nail will try to classify such MIME message parts on its own, if
                possible, and through their file name.  This variable can also be given a non-empty value, in
                which case the value is expected to be a number, actually a carrier of bits.  Creating the bit-
                carrying number is a simple addition:

                      ? !echo Value should be set to $((2 + 4))
                      Value should be set to 6

                   If bit two is set (2) then the detected content-type will be carried along with the message
                    and be used for deciding which pipe-TYPE/SUBTYPE is responsible for the MIME part, shall
                    that question arise; 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 (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.

     mimetypes-load-control
                This option can be used to control which of the mime.types(5) databases are loaded by S-nail, as
                furtherly described in the section The mime.types files.  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; the user file is
                loaded first, letter matching is case-insensitive.  If this option is not set S-nail will try to
                load both files instead.  Incorporation of the S-nail-builtin MIME types cannot be suppressed,
                but they will be matched last.

                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.

     NAIL_EXTRA_RC
                The name of an optional startup file to be read after ~/.mailrc.  This variable has an effect
                only if it is set in s-nail.rc or ~/.mailrc, it is not imported from the environment in order to
                honour ‘MAILRC=/dev/null/’ -n invocations.  Use this file for commands that are not understood
                by other POSIX mailx(1) implementations.

     NAIL_HEAD  A string to put at the beginning of each new message.  The escape sequences tabulator ‘\t’ and
                newline ‘\n’ are understood.

     NAIL_HISTFILE
                [Option] If a command line editor is available then this can be set to name the (expandable)
                path of the location of a permanent history file.

     NAIL_HISTSIZE
                [Option] If a command line editor is available this value restricts the amount of history
                entries that are saved into a set and valid NAIL_HISTFILE.  A value of less than 0 disables this
                feature; note that loading and incorporation of NAIL_HISTFILE upon program startup can also be
                suppressed by doing this.  An unset or invalid value, or 0, causes a default value to be used.
                Dependent on the available command line editor this will also define the number of history
                entries in memory; it is also editor-specific wether runtime updates of this value will be
                honoured.

     NAIL_TAIL  A string to put at the end of each new message.  The escape sequences tabulator ‘\t’ and newline
                ‘\n’ are understood.

     newfolders
                If this variable has the value ‘maildir’, newly created local folders will be in Maildir format.

     newmail    Checks for new mail in the current folder each time the prompt is printed.  For IMAP mailboxes
                the server is then polled for new mail, which may result in delayed operation if the connection
                to the server is slow.  A Maildir folder must be re-scanned to determine if new mail has
                arrived.

                If this variable is set to the special value “nopoll” an IMAP server is not actively asked for
                new mail, but new mail may still be detected and announced with any other IMAP command that is
                sent to the server.  In either case the IMAP server may send notifications about messages that
                have been deleted on the server by another process or client.  In this case, “Expunged X
                messages” is printed regardless of this variable, and message numbers may have changed.

                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.

     ORGANIZATION
                The value to put into the ‘Organization:’ field of the message header.

     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.

     pipe-TYPE/SUBTYPE
                When a MIME message part of type ‘TYPE/SUBTYPE’ (normalized to lowercase) is displayed or
                quoted, its text is filtered through the value of this variable interpreted as a shell command.
                The special value ‘@’ can be used to force interpretation of the message part as plain text,
                e.g., ‘set pipe-application/pgp-signature=@’ will henceforth treat signatures as plain text and
                display them "as is".  (The same can also be achieved, in a more useful context, by using the
                mimetype command in conjunction with a type marker.)

                Also, if a shell command is prefixed with ‘@’, then the command will only be used to prepare the
                MIME message part if the message is displayed by itself, but not when multiple messages are
                displayed at once.

                Finally, if a shell command is prefixed with ‘@&’, then, in addition to what has been described
                for the plain ‘@’ shell command prefix, the command will be run asynchronously, i.e., without
                blocking S-nail, which may be a handy way to display a, e.g., PDF file while also continuing to
                read the mail message.  Some information about the MIME part to be displayed is embedded into
                the environment of the shell command:

                NAIL_TMPDIR              The temporary directory that S-nail uses.  Usually identical to TMPDIR,
                                         but guaranteed to be set and usable by child processes; to ensure the
                                         latter condition for TMPDIR also, it'll be set.
                NAIL_FILENAME            The filename, if any is set, the empty string otherwise.
                NAIL_FILENAME_GENERATED  A random string.
                NAIL_CONTENT             The MIME content-type of the part, if known, the empty string
                                         otherwise.
                NAIL_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 NAIL_CONTENT otherwise.

     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-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.

     prompt     The string printed when a command is accepted.  Prompting may be prevented by either setting
                this to the null string or by setting noprompt.  The same XSI escape sequences that are
                understood by the echo command may be used within prompt.

                In addition, the following S-nail specific additional sequences are understood: ‘\&’, which
                expands to “?” unless bsdcompat is set, in which case it expands to “&”; note that ‘\& ’ is the
                default value of prompt.  ‘\?’, which will expand to “1” if the last command failed and to “0”
                otherwise, ‘\$’, which will expand to the name of the currently active account, if any, and to
                the empty string otherwise, and ‘\@’, which will expand to the name of the currently active
                mailbox.  (Note that the prompt buffer is size-limited, excess is cut off.)

                Even though prompt checks for headline-bidi to encapsulate the expansions of the ‘\$’ and ‘\@’
                escape sequences as necessary to correctly display bidirectional text, this is not true for the
                final string that makes up prompt as such, i.e., real BIDI handling is not supported.

                When a newer version of the editline(3) Command line editor is used, any escape sequence must
                itself be encapsulated with another escape character for usage with the EL_PROMPT_ESC mechanism:
                S-nail configures the control character ‘\01’ for this.

     quote      If set, S-nail starts a replying message with the original message prefixed by the value of the
                variable indentprefix.  Normally, a heading consisting of “Fromheaderfield wrote:” is printed
                before the quotation.  If the string ‘noheading’ is assigned to the quote variable, this heading
                is omitted.  If the string ‘headers’ is assigned, the headers selected by the ignore/retain
                commands are printed above the message body, thus quote acts like an automatic `~m' TILDE
                ESCAPES command, then.  If the string ‘allheaders’ is assigned, all headers are printed above
                the message body and all MIME parts are included, making quote act like an automatic `~M'
                command; also see quote-as-attachment.

     quote-fold
                [Option] Can be set in addition to indentprefix.  Setting this turns on a more fancy quotation
                algorithm in that leading quotation characters are compressed and overlong lines are folded.
                quote-fold can be set to either one or two (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-, not paragraph-based.  If not set explicitly the minimum
                will reflect the goal algorithmically.  The goal can't be smaller than the length of
                indentprefix plus some additional pad.  Necessary adjustments take place silently.

     record     If defined, gives the pathname of the folder used to record all outgoing mail.  If not defined,
                then outgoing mail is not saved.  When saving to this folder fails the message is not sent, but
                instead saved to DEAD.

     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 builtin strings as ‘Subject:’ reply message
                indicators – builtin are ‘Re:’, which is mandated by RFC 5322, as well as the german ‘Aw:’.

     replyto    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.

     reply-to-honour
                Controls wether 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”.

     screen     When S-nail initially prints the message headers it determines the number to print by looking at
                the speed of the terminal.  The faster the terminal, the more it prints.  This option overrides
                this calculation and specifies how many message headers are printed.  This number is also used
                for scrolling with the z command.

     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.

     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.  The sender address is handled as if it were in the alternates list.

     sendmail   To use an alternate mail transport agent (MTA), set this option to the full pathname of the
                program to use.  It may be necessary to set sendmail-progname in addition.

                The MTA will be passed command line arguments from several possible sources: from the variable
                sendmail-arguments if set, from the command line if given and the variable expandargv allows
                their use.  Argument processing of the MTA will always be terminated with a ‘--’ separator.

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

     sendmail-arguments
                Arguments to pass through to the Mail-Transfer-Agent can be given via this option.  The content
                of this variable will be split up in a vector of arguments which will be joined onto other
                possible MTA options:

                      set sendmail-arguments='-t -X "/tmp/my log"'

     sendmail-no-default-arguments
                (Binary) Unless this option is set S-nail will pass some well known standard command line
                options to the defined sendmail program, see there for more.

     sendmail-progname
                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 “sendmail”) will treat its contents as that name.  The default is ‘sendmail’.

     Sign       A string for use with the ~A tilde escape.

     sign       A string for use with the ~a tilde escape.

     signature  Must correspond to the name of a readable file if set.  The file's content is then appended to
                each singlepart message and to the first part of each multipart message.  Be warned that there
                is no possibility to edit the signature for an individual message.

     smime-ca-dir
                [Option] Specifies a directory with CA certificates in PEM (Privacy Enhanced Mail) format for
                verification of S/MIME signed messages.

     smime-ca-file
                [Option] Specifies a file with CA certificates in PEM format for verification of 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’ isn't
                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-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 as well as 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.

     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 is most useful for long certificate chains if
                it is desired to aid the receiving party's verification process.  Note that top level
                certificates may also be included in the chain but don't play a role for verification.  Also see
                smime-sign-cert.  Remember that for this ‘USER@HOST’ refers to the variable from (or, if that
                contains multiple addresses, sender).

     smime-sign-message-digest-USER@HOST, smime-sign-message-digest
                [Option] Specifies the message digest to use when signing S/MIME messages.  RFC 5751 mandates a
                default of ‘sha1’.  Possible values are (case-insensitive and) in decreasing cipher strength:
                ‘sha512’, ‘sha384’, ‘sha256’, ‘sha224’ and ‘md5’.

                The actually available message digest algorithms depend on the cryptographic library that S-nail
                uses.  [Option] Support for more message digest algorithms may be available through dynamic
                loading via, e.g., EVP_get_digestbyname(3) (OpenSSL) if S-nail has been compiled to support
                this.  Remember that for this ‘USER@HOST’ refers to the variable from (or, if that contains
                multiple addresses, sender).

     smtp       [Option] Normally S-nail invokes the program defined via sendmail to transfer messages, as
                described in Sending mail.  Setting the smtp variable will instead cause SMTP network
                connections be made to the server specified therein in order to directly submit the message.
                S-nail knows about three different “SMTP protocols”:

                   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 SSL/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.
                   Then the so-called SMTPS which is supposed to live on server port 465 and is automatically
                    SSL/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.
                   Finally there is the SUBMISSION protocol (RFC 6409), which usually lives on server port 587
                    and is practically identically to the SMTP protocol from S-nails point of view beside that;
                    it requires setting the smtp-use-starttls variable to enter a SSL/TLS secured session state.
                    Assign a value like [v15-compat] ‘submission://[user[:password]@]server[:port]’ ([no
                    v15-compat] ‘submission://server[:port]’).

                For more on credentials etc. please see On URL syntax and credential lookup.  The SMTP transfer
                is executed in a child process, which runs asynchronously unless either the sendwait or the
                verbose variable is set.  If it receives a TERM signal, it will abort and save the message to
                DEAD.

     smtp-auth-USER@HOST, smtp-auth-HOST, smtp-auth
                [Option] Variable chain that sets the SMTP 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 doesn't need any user credentials, ‘gssapi’
                requires a user name and all other methods require a user name and a password.  See [v15-compat]
                smtp, 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
                dependend 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
                [v15-compat] Normally S-nail uses the variable from to derive the necessary ‘USER@HOST’
                information to issue a ‘MAIL FROM:<>’ SMTP command.  Setting smtp-hostname can be used to use
                the ‘USER’ from the SMTP account (smtp 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
                the generated ‘Message-ID:’.

     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) (SpamAssassin:
                          http://spamassassin.apache.org) 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 doesn't inspect the
                          ‘is-spam’ flag of a message for the command spamforget.

                ‘spamd’   S-nail will directly communicate with the spamassassin(1) daemon spamd(1) via a path-
                          based unix(4) stream socket as specified in spamd-socket.  It is possible to specify a
                          per-user configuration via spamd-user.

                ‘filter’  generic spam filter support via freely configurable hooks.  This interface is ment for
                          programs like bogofilter(1) and sylfilter(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 variables NAIL_TMPDIR, TMPDIR and NAIL_FILENAME_GENERATED set.
                          Note that spam score support for spamrate isn't 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 won't be passed through to the configured
                spam-interface.  The default is 420000 bytes.

     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 specifiy 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.

     spamd-socket
                [Option] Specify the path of the unix(4) domain socket on which spamd(1) listens for connections
                for the ‘spamd’ spam-interface.  Note that the path is not expanded, but used “as is”.

     spamd-user
                [Option] Specify a username for per-user configuration files for the ‘spamd’ 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]tional 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 a
                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
                [Option] Specifies a directory with CA certificates in PEM (Pricacy Enhanced Mail) for
                verification of of SSL/TLS server certificates.  See SSL_CTX_load_verify_locations(3) for more
                information.

     ssl-ca-file
                [Option] Specifies a file with CA certificates in PEM format for verification of SSL/TLS server
                certificates.  See SSL_CTX_load_verify_locations(3) for more information.

     ssl-cert-USER@HOST, ssl-cert-HOST, ssl-cert
                [Option] Variable chain that sets the file name for a SSL/TLS client certificate required by
                some servers.  This is a direct interface to the ‘Certificate’ slot of the SSL_CONF_cmd(3)
                function of the OpenSSL library, if available.

     ssl-cipher-list-USER@HOST, ssl-cipher-list-HOST, ssl-cipher-list
                [Option] Specifies a list of ciphers for SSL/TLS connections.  This is a direct interface to the
                ‘CipherString’ slot of the SSL_CONF_cmd(3) function of the OpenSSL library, if available; see
                ciphers(1) for more information.  By default S-nail doesn't set a list of ciphers, which in
                effect will use a ssl-protocol specific cipher (protocol standards ship with a list of
                acceptable ciphers), possibly cramped to what the actually used SSL/TLS library supports – the
                manual section An example configuration also contains a SSL/TLS use case.

     ssl-config-file
                [Option] If this variable is set S-nail will call CONF_modules_load_file(3) to allow OpenSSL to
                be configured according to the host system wide security settings.  If a non-empty value is
                given then this will be used to specify the configuration file to be used instead of the global
                OpenSSL default; note that in this case it is an error if the file cannot be loaded.  The
                application name will always be passed as “s-nail”.

     ssl-crl-file
                [Option] Specifies a file that contains a CRL in PEM format to use when verifying SSL/TLS server
                certificates.

     ssl-crl-dir
                [Option] Specifies a directory that contains files with CRLs in PEM format to use when verifying
                SSL/TLS server certificates.

     ssl-key-USER@HOST, ssl-key-HOST, ssl-key
                [Option] Variable chain that sets the file name for the private key of a SSL/TLS client
                certificate.  If unset, the name of the certificate file is used.  The file is expected to be in
                PEM format.  This is a direct interface to the ‘PrivateKey’ slot of the SSL_CONF_cmd(3) function
                of the OpenSSL library, if available.

     ssl-method-USER@HOST, ssl-method-HOST, ssl-method
                [Option] [Obsolete] Please use the newer and more flexible ssl-protocol instead: if both values
                are set, ssl-protocol will take precedence!  Can be set to the following values, the actually
                used ssl-protocol specification to which it is mapped is shown in parenthesis: ‘tls1.2’ (‘-ALL,
                TLSv1.2’), ‘tls1.1’ (‘-ALL, TLSv1.1’), ‘tls1’ (‘-ALL, TLSv1’) and ‘ssl3’ (‘-ALL, SSLv3’); the
                special value ‘auto’ is mapped to ‘ALL, -SSLv2’ and thus includes the SSLv3 protocol.  Note that
                SSLv2 is no longer supported at all.

     ssl-protocol-USER@HOST, ssl-protocol-HOST, ssl-protocol
                [Option] Specify the used SSL/TLS protocol.  This is a direct interface to the ‘Protocol’ slot
                of the SSL_CONF_cmd(3) function of the OpenSSL library, if available; otherwise an S-nail
                internal parser is used which understands the following subset of (case-insensitive) command
                strings: ‘SSLv3’, ‘TLSv1’, ‘TLSv1.1’ and ‘TLSv1.2’, as well as the special value ‘ALL’.
                Multiple specifications may be given via a comma-separated list which ignores any whitespace.
                An optional ‘+’ plus prefix will enable a protocol, a ‘-’ minus prefix will disable it, so that
                ‘-ALL, TLSv1.2’ will enable only the TLSv1.2 protocol.

                It depends upon the used TLS/SSL library which protocols are actually supported and which
                protocols are used if ssl-protocol is not set, but note that SSLv2 is no longer supported at all
                and actively disabled.  Especially for older protocols explicitly securing ssl-cipher-list may
                be worthwile, see An example configuration.

     ssl-rand-egd
                [Option] Gives the pathname to an entropy daemon socket, see RAND_egd(3).  Not all SSL/TLS
                libraries support this.

     ssl-rand-file
                [Option] Gives the pathname to a file with entropy data, see RAND_load_file(3).  If the file is
                a regular file writable by the invoking user, new data is written to it after it has been
                loaded.

     ssl-verify-USER@HOST, ssl-verify-HOST, ssl-verify
                [Option] Variable chain that sets the action to be performed if an error occurs during SSL/TLS
                server certificate validation.  Valid (case-insensitive) values are ‘strict’ (fail and close
                connection immediately), ‘ask’ (ask whether to continue on standard input), ‘warn’ (print a
                warning and continue), ‘ignore’ (do not perform validation).  The default is ‘ask’.

     stealthmua
                If only set without an assigned value, then this option inhibits the generation of the
                ‘Message-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:’ suppression doesn't occur.

     toplines   If defined, gives the number of lines of a message to be printed out with the top command;
                normally, the first five lines are printed.

     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 unless it can deduce a value from the LC_CTYPE locale
                environment.  Refer to the section Character sets for the complete picture about character sets.

     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 value of
                USER.

     version, version-major, version-minor, version-update
                (Read-only) S-nail version information: the first variable contains a string containing the
                complete version identification – this is identical to the output of the command version.  The
                latter three contain only digits: the major, minor and update version numbers.

ENVIRONMENT

     The term “environment variable” should be considered an indication that the following variables are either
     standardized as being vivid parts of process environments, or are commonly found in there.  Unless
     otherwise explicitly noted they integrate into the normal variable handling, as documented above, from
     S-nails point of view.

     COLUMNS   The user's preferred width in column positions for the terminal screen or window.  Queried and
               used once on program startup.

     DEAD      The name of the file to use for saving aborted messages if save is set; this defaults to
               dead.letter in the user's HOME directory.

     EDITOR    Pathname of the text editor to use in the edit command and ~e TILDE ESCAPES.  A default editor is
               used if this value is not defined.

     HOME      The user's home directory.  This variable is only used when it resides in the process
               environment.  Use setenv to update the value at runtime.

     LANG, LC_ALL, LC_COLLATE, LC_CTYPE, LC_MESSAGES
               See locale(7) and Character sets.

     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.

     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).

     MBOX      The name of the user's mbox file.  Supports a logical subset of the special conventions that are
               documented for the file command and the folder option.  The fallback default is mbox in the
               user's HOME directory.

     MAILRC    Is used as a startup file instead of ~/.mailrc if set.  When S-nail scripts are invoked on behalf
               of other users, this variable should be set to /dev/null to avoid side-effects from reading their
               configuration files.  This variable is only used when it resides in the process environment.

     NAIL_NO_SYSTEM_RC
               If this variable is set then reading of s-nail.rc at startup is inhibited, i.e., the same effect
               is achieved as if S-nail had been started up with the option -n.  This variable is only used when
               it resides in the process environment.

     NETRC     [v15-compat] [Option] This variable overrides the default location of the user's .netrc file.

     PAGER     Pathname of the program to use in the more command or when the crt variable is set.  The default
               paginator is more(1) (path search through SHELL).

     PATH      A list of directories that is searched by the shell when looking for commands (as such only
               recognized in the process environment).

     SHELL     The shell to use for the commands !, shell, the ~! TILDE ESCAPES and when starting subprocesses.
               A default shell is used if this option is not defined.

     SYSV3     Changes the letters printed in the first column of a header summary.

     TERM      [Option] The terminal type for which output is to be prepared.

     TMPDIR    Used as directory for temporary files instead of /tmp, if set.  This variable is only used when
               it resides in the process environment.  Use setenv to update the value at runtime.

     USER      Force identification as the given user, i.e., identical to the -u command line option.  This
               variable is only used when it resides in the process environment.  Use setenv to update the value
               at runtime, but note that doing so won't trigger any of those validation checks that were
               performed on program startup (again).

     VISUAL    Pathname of the text editor to use in the visual command and ~v TILDE ESCAPES.

FILES

     ~/.mailrc        File giving initial commands.

     s-nail.rc        System wide initialization file.

     ~/.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 users .netrc file – the section The
                      .netrc file documents the file format.

   The mime.types files
     When sending messages S-nail tries to determine the content type of all attachments.  When displaying
     message content or attachments S-nail uses the content type to decide wether it can directly display data
     or wether it needs to deal with content handlers, as can be defined via pipe-TYPE/SUBTYPE (and
     pipe-EXTENSION) variables, to do so.

     It learns about MIME types and how to treat them by reading mime.types files, the loading of which can be
     controlled by setting the variable mimetypes-load-control.  (The command mimetype can also be used to deal
     with MIME types.)  mime.types files have the following syntax:

           type/subtype extension [extension ...]

     where ‘type/subtype’ are strings describing the file contents, and one or multiple ‘extension’s, separated
     by whitespace, name the part of a filename starting after the last dot (of interest).  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 specially 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.

     Further reading: for sending messages: mime-allow-text-controls.  For reading etc. messages: Viewing HTML
     mail and MIME attachments, mime-counter-evidence, pipe-TYPE/SUBTYPE, pipe-EXTENSION.

   The .netrc file
     The .netrc file contains user credentials for machine accounts.  The default location in the user's HOME
     directory may be overridden by the NETRC environment variable.  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 doesn't support single, but only double quotation marks, e.g., ‘password="pass with spaces"’.
        BSD (only?) supports escaping of single characters via a backslash (e.g., a space can be escaped via
         ‘\ ’), in- as well as outside of a quoted string.
        BSD doesn't require the final quotation mark of the final user input token.
        At least Hewlett-Packard seems to support a format which also allows tokens to be separated with commas
         – this format is not supported!
        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:

     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 shouldn't 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

           # Where are the up-to-date SSL certificates?
           #set ssl-ca-dir=/etc/ssl/certs
           set ssl-ca-file=/etc/ssl/certs/ca-certificates.crt

           # (Since we manage up-to-date ones explicitly, don't use any,
           # possibly outdated, default certificates shipped with OpenSSL
           set ssl-no-default-ca

           # Don't use protocols olders than TLS v1.2.
           # Change this only when the remote server doesn't support it:
           # maybe use ssl-protocol-HOST (or -USER@HOST) syntax to define
           # such explicit exceptions, then
           set ssl-protocol="-ALL,+TLSv1.2"

           # Explicitly define the list of ciphers, which may improve security,
           # especially with protocols older than TLS v1.2.  See ciphers(1).
           # Hint: it is important to include "@STRENGTH": only with it the
           # final list will be sorted by algorithm strength.
           # This is an example: in reality it is possibly best to only use
           # ssl-cipher-list-HOST (or -USER@HOST), as necessary, again..
           set ssl-cipher-list="ALL:!aNULL:!MEDIUM:!LOW:\
               !MD5:!RC4:!EXPORT:@STRENGTH"

           # Request strict transport security checks!
           set ssl-verify=strict

           # 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 to or forwarding a message the comment and name
           # parts of email addresses are removed unless this variable is set
           set fullnames

           # When sending messages, wait until the Mail-Transfer-Agent finishs.
           # Only like this you'll be able to see errors reported through the
           # exit status of the MTA (including the builtin SMTP one)!
           set sendwait

           # Only use builtin MIME types, no mime.types(5) files
           set mimetypes-load-control

           # Default directory where we act in (relative to $HOME)
           set folder=mail MBOX=+mbox.mbox record=+sent.mbox \
               DEAD=+dead.mbox

           # 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 <youremail@domain>"

           # It may be necessary to set hostname and/or smtp-hostname
           # if the "SERVER" of smtp and "domain" of from don't match.
           # The `urlencode' command can be used to encode USER and PASS
           set smtp=(smtp[s]/submission)://[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 \
               history-gabby mime-counter-evidence=6 \
               prompt="\033[31m?\?[\$ \@]\& \033[0m" \
               NAIL_HISTFILE=+.s-nailhist NAIL_HISTSIZE=-1 \
               reply-to-honour=ask-yes

           # When `p'rinting messages, show only these headers
           # (use `P'rint for all headers and `S'how for raw message)
           retain date from to cc subject

           # Some mailing lists
           mlist @xyz-editor.xyz$ @xyzf.xyz$
           mlsubscribe ^xfans@xfans.xyz$

           # A real life example of a very huge free mail provider
           account XooglX {
             localopts yes
             set from="Your Name <youremail@domain>"
             # (The plain smtp:// proto is optional)
             set smtp=USER:PASS@smtp.gmXil.com smtp-use-starttls
           }

           # 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
           account XandeX {
             localopts true
             set from="Your Name <youremail@domain>"
             set smtp=smtps://USER:PASS@smtp.yaXXex.ru:465 \
                 hostname=yaXXex.com smtp-hostname=
           }

           # Create some new commands so that, e.g., `ls /tmp' will..
           ghost l   !ls -aFtr
           ghost L   !ls -aFt
           ghost ll  !ls -aFltr
           ghost Ll  !ls -aFlt
           ghost la  !ls -aFr
           ghost La  !ls -aF
           ghost lla !ls -aFlr
           ghost Lla !ls -aFl
           ghost lS  !ls -aFrS
           ghost LS  !ls -aFS
           ghost llS !ls -aFlrS
           ghost LlS !ls -aFlS

           # We don't support gpg(1) directly yet.  But simple --clearsign'd
           # message parts can be dealt with as follows:
           define V {
             localopts yes
             set pipe-text/plain="set -C;\
               : > \"${TMPDIR}/${NAIL_FILENAME_GENERATED}\" \
               trap \"rm -f \\\"${TMPDIR}/${NAIL_FILENAME_GENERATED}\\\"\" \
                 EXIT INT QUIT PIPE TERM;\
               set +C;\
               cat > \"${TMPDIR}/${NAIL_FILENAME_GENERATED}\";\
               < \"${TMPDIR}/${NAIL_FILENAME_GENERATED}\" awk \
                   -v TMPFILE=\"${TMPDIR}/${NAIL_FILENAME_GENERATED}\" '\
                 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
           }
           ghost V call V

           define RK {
             !printf 'Key IDs to gpg --recv-keys: ';\
               read keyids;\
               gpg --recv-keys ${keyids};
           }
           ghost RK call RK

     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:

           account XandeX {
             localopts true
             set from="Your Name <youremail@domain>"
             set netrc-lookup
             #set agent-shell-lookup="gpg -d .pass.gpg"

             set smtp=smtps://smtp.yXXXXx.ru:465 \
                 smtp-hostname= hostname=yXXXXx.com
             set pop3-keepalive=240 pop3-no-apop-pop.yXXXXx.ru
             ghost xp fi pop3s://pop.yXXXXx.ru
           }

     and, in the .netrc file:

           machine *.yXXXXx.ru login USER password PASS

     If the also [Option]al agent-shell-lookup is available things could be diversified further by using
     encrypted password storage: for this, don't specify ‘password PASS’ in the .netrc file and instead
     uncomment the line that defines agent lookup in the example account above, then create the encrypted
     password storage file .pass.gpg:

           $ echo PASS > .pass
           $ gpg -e .pass
           $ eval `gpg-agent --daemon \
                   --pinentry-program=/usr/bin/pinentry-curses \
                   --max-cache-ttl 99999 --default-cache-ttl 99999`

     This configuration should now work just fine (use the -d command line option for a(n almost) dry-run):

           $ echo text | s-nail -vv -AXandeX -s Subject some@where

   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 was
     sent using a valid certificate, that the sender's address in the message header 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 recipient's 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 recipient's 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.  A message should always be signed before it is encrypted.
     Otherwise, it is still possible that the encrypted message text is altered.

     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 with OpenSSL and installed on your system.  If you trust the source of your OpenSSL software
     installation, this offers reasonable security for S/MIME on the Internet.  (Otherwise set ssl-no-default-ca
     and use smime-ca-file and/or smime-ca-dir.)  In general, a certificate cannot be more secure than the
     method its CA certificate has been retrieved with, though.  Thus if you download a CA certificate from the
     Internet, you can only trust the messages you verify using that certificate as much as you trust the
     download process.

     The first thing you need for participating in S/MIME message exchange is your personal certificate,
     including a private key.  The certificate contains public information, in particular your name and your
     email address(es), and the public key that is used by others to encrypt messages for you, and to verify
     signed messages they supposedly received from you.  The certificate is included in each signed message you
     send.  The private key must be kept secret.  It is used to decrypt messages that were previously encrypted
     with your public key, and to sign messages.

     For personal use it is recommended that you get a S/MIME certificate from one of the major CAs on the
     Internet using your WWW browser.  Many CAs offer such certificates for free.  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 you will have to download their root certificate separately and
     ensure it is part of our S/MIME certificate validation chain by including it in smime-ca-dir or as a vivid
     member of the smime-ca-file.  But let's 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's 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 you will have to create a combined private key/public key
     (certificate) file:

           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.  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-message-digest=SHA256 \
             smime-sign

     From each signed message you send, the recipient can fetch your certificate and use it to send encrypted
     mail back to you.  Accordingly if somebody sends you a signed message, you can do the same, and use the
     verify command to check the validity of the certificate.

     Options of interest for S/MIME signing: smime-ca-dir, smime-ca-file, smime-crl-dir, smime-crl-file,
     smime-no-default-ca, smime-sign, smime-sign-cert, smime-sign-include-certs and smime-sign-message-digest.

     After it has been verified save the certificate via certsave and tell S-nail that it should use it for
     encryption for further communication with that somebody:

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

     Additional options of interest for S/MIME en- and decryption: smime-cipher and smime-encrypt-USER@HOST.

     You should carefully consider if you prefer to store encrypted messages in decrypted form.  If you do,
     anybody who has access to your mail folders can read them, but if you do not, you might be unable to read
     them yourself later if you happen to lose your private key.  The decrypt command saves messages in
     decrypted form, while the save, copy, and move commands leave them encrypted.

     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.

   Using CRLs with S/MIME or SSL/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 SSL/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 you have to retrieve them 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 ssl-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.

   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.  Once messages have been identified as
     spam their (volatile) ‘is-spam’ state can be prompted: the ‘:s’ and ‘:S’ message specifications will
     address respective messages and their attrlist entries will be used when displaying the headline in the
     header display.

        spamrate rates the given messages and sets their ‘is-spam’ flag accordingly.  If the spam interface
         offers spam scores those can also be displayed in the header display by including the ‘%$’ format in
         the headline variable.

        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-interfaces ‘spamc’ and ‘spamd’ require 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’ only works via a local path-based unix(4) socket, but otherwise the following will be
     equivalently fine:

           $ 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=spamd -Sspam-maxsize=500000 \
               -Sspamd-socket=/tmp/.spamsock -Sspamd-user=

           $ 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) and sylfilter(1).  Here is an
     example for the former, 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:

           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-FOLDER=spamdelhook

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

FAQ

   S-nail shortly hangs on startup
     This can have two reasons, one is the necessity to wait for a file lock and can't 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).  You 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 what you'd expect?

   I can't login to Google mail a.k.a. GMail
     Since 2014 some free service providers classify programs as “less secure” unless they use a special
     authentification method (OAuth 2.0) which wasn't 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 has to be performed whenever S-nail is invoked from the command
     line (in interactive sessions situation may differ).

     S-nail doesn't 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 your real Google account password in S-nail (for more on that see
          the section On URL syntax and credential lookup).

SEE ALSO

     bogofilter(1), bzip2(1), file(1), fmt(1), gpg(1), gpg-agent(1), gzip(1), less(1), more(1), newaliases(1),
     openssl(1), postfix(1), printf(1), sendmail(1), sh(1), spamassassin(1), spamc(1), spamd(1), sylfilter(1),
     vacation(1), xterm(1), xz(1), editline(3), iconv(3), readline(3), setlocale(3), ssl(3), aliases(5),
     locale(7), mailaddr(7), re_format(7), exim(8), mailwrapper(8), sendmail(8)

HISTORY

     A mail command appeared in Version 1 AT&T Unix.  Berkeley Mail was written in 1978 by Kurt Shoens.  This
     man page is derived from from “The Mail Reference Manual” originally written by Kurt Shoens.  “Heirloom
     Mailx” enhancements are maintained and documented by Gunnar Ritter.  “S-nail” is maintained and documented
     by Steffen (Daode) Nurpmeso.

     Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition,
     Standard for Information Technology – Operating System Interface (POSIX), The Open Group Base
     Specifications Issue 6, Copyright © 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc
     and The Open Group.  In the event of any discrepancy between this version and the original IEEE and The
     Open Group Standard, the original IEEE and The Open Group Standard is the referee document.  The original
     Standard can be obtained online at OpenGroup.org: http://www.opengroup.org/unix/online.html Redistribution
     of this material is permitted so long as this notice remains intact.

AUTHORS

     Kurt Shoens,
     Christos Zoulas,
     Gunnar Ritter,
     Steffen Nurpmeso <s-nail-users@lists.sourceforge.net>

CAVEATS

     The character set conversion uses and relies upon the iconv(3) function.  Its functionality differs widely
     between the various system environments S-nail runs on.

     Limitations with IMAP mailboxes are: It is not possible to edit messages, but it is possible to append
     them.  Thus to edit a message, create a local copy of it, edit it, append it, and delete the original.  The
     line count for the header display is only appropriate if the entire message has been downloaded from the
     server.  The marking of messages as `new' is performed by the IMAP server; use of the exit command instead
     of quit will not cause it to be reset, and if the newmail variable is unset, messages that arrived during a
     session will not be in state `new' anymore when the folder is opened again.  Also if commands queued in
     disconnected mode are committed, the IMAP server will delete the `new' flag for all messages in the changed
     folder, and new messages will appear as unread when it is selected for viewing later.  The `flagged',
     `answered', and `draft' attributes are usually permanent, but some IMAP servers are known to drop them
     without notification.  Message numbers may change with IMAP every time before the prompt is printed if
     S-nail is notified by the server that messages have been deleted by some other client or process.  In this
     case, `Expunged n messages' is printed, and message numbers may have changed.

     Limitations with POP3 mailboxes are: It is not possible to edit messages, they can only be copied and
     deleted.  The line count for the header display is only appropriate if the entire message has been
     downloaded from the server.  The status field of a message is maintained by the server between connections;
     some servers do not update it at all, and with a server that does, the exit command will not cause the
     message status to be reset.  The newmail command and the newmail variable have no effect.  It is not
     possible to rename or to remove POP3 mailboxes.

     If a “RUBOUT” (interrupt, ‘control-C’) is typed while an IMAP or POP3 operation is in progress, S-nail will
     wait until the operation can be safely aborted, and will then return to the command loop and print the
     prompt again.  When a second “RUBOUT” is typed while S-nail is waiting for the operation to complete, the
     operation itself will be cancelled.  In this case, data that has not been fetched yet will have to be
     fetched before the next command can be performed.  If the cancelled operation was using an SSL/TLS
     encrypted channel, an error in the SSL transport will very likely result and render the connection
     unusable.

     As S-nail is a mail user agent, it provides only basic SMTP services.  If it fails to contact its upstream
     SMTP server, it will not make further attempts to transfer the message at a later time, and it does not
     leave other information about this condition than an error message on the terminal and an entry in DEAD.
     This is usually not a problem if the SMTP server is located in the same local network as the computer on
     which S-nail is run.  However, care should be taken when using a remote server of an ISP; it might be
     better to set up a local SMTP server then which just acts as a proxy.

     S-nail immediately contacts the SMTP server (or sendmail(1)) even when operating in disconnected mode.  It
     would not make much sense for S-nail to defer outgoing mail since SMTP servers usually provide much more
     elaborated delay handling than S-nail could perform as a client.  Thus the recommended setup for sending
     mail in disconnected mode is to configure a local SMTP server such that it sends outgoing mail as soon as
     an external network connection is available again, i.e., to advise it to do that from a network startup
     script.

BUGS

     With IMAP, at least if the IMAP cache is used, if multiple connect and disconnect cycles happen without an
     intervening change of the active mailbox then S-nail will at some time loose the ability to keep the local
     state up-to-date, meaning that, e.g., messages show up with false numbers, and including the possibility
     that messages are accessed via numbers that are no(t longer) valid, resulting in program crashes.  The
     solution is to change the active mailbox before that happens :).  Also see the file TODO from the
     distribution or the repository.