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)
                          (http://spamassassin.apache.org SpamAssassin) suite.  Different to the
                          generic filter interface S-nail will automatically add the correct
                          arguments for a given command and has the necessary knowledge to parse
                          the program's output.  A default value for spamc-command will have been
                          compiled into the S-nail binary if spamc(1) has been found in PATH
                          during compilation.  Shall it be necessary to define a specific
                          connection type (rather than using a configuration file for that), the
                          variable spamc-arguments can be used as in, e.g., ‘-d
                          server.example.com -p 783’.  It is also possible to specify a per-user
                          configuration via spamc-user.  Note that this interface 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.