==== SSH session that runs sendmail instead of an interactive shell ====

=== What? ===
A wrapper for your local mail transfer agent (MTA) that initiates an SSH session when your SDF email address appears in the "From:" header.

=== Why? ===
You want to send email from your SDF address without running the risk that the receiving server will reject it due to a mismatch between the "From" header and your ISP's IP block, but ...

  * the low volume of your outgoing mail cannot justify the cost of a VPM/VHOST/MetaARPA membership, or
  * your Internet connection has too high a latency to be running a full-screen editor within an SSH session.

=== Where? ===
a *BSD, Linux, or WSL environment with a client that generates MIME-formatted mail (as expected by sendmail-compatible MTAs such as msmtp)

=== How? ===
  - Set up [[ssh public key authentication|SSH public key authentication]], if you have not already done so. This step can be omitted if you prefer to type your password for each outgoing mail.
  - Save the wrapper script below somewhere in your $PATH. Change the value of $MTA to the mail transport agent that you would otherwise be using for outgoing mail, the value of $PATH if any of the commands live in non-standard locations, and of course all the hard-coded email addresses in the script.
  - Configure your mail user agent (MUA) to call the wrapper script, rather than calling your MTA directly.

<code>
#!/usr/bin/env bash
#
# msmtp.wrap - bypass local in favor of remote sendmail,
#              if certain headers are found

PATH=/bin:/usr/bin:/usr/local/bin
Account0=me@sdf.org
Account1=me@other-pubnix.net
MTA=msmtp
draft="$(mktemp -t sendmail.XXXXXX)"
cat /dev/stdin > "$draft"
sender="$(grep '^From:' "$draft" | head -n 1 | cut -d: -f2)"

case "$sender" in
    *$Account0*)
        sendmail="ssh $Account0 sendmail -t" ;;
    *$Account1*)
        sendmail="ssh $Account1 sendmail -t" ;;
    *)
        sendmail="$MTA $*" ;;
esac

< "$draft" $sendmail; status=$?
rm -f "$draft"; exit $status
</code>

=== What Next? ===

The possibilities for customization of this wrapper are only limited by your imagination. You can parse the outgoing messages to look for particular recipients, subject lines, etc., and define a custom $sendmail command to handle each case. On the SDF side, the arguments following ''ssh $Account0'' will be passed to your usual login shell after the ''-c'' option(( see the sshd(8) man-page, [[ https://man.openbsd.org/sshd ]] )), so you will enjoy all the environment variables defined in ''$HOME/.bash_login'' if you use bash as your login shell. Save your custom scripts in a directory that your SDF login shell considers part of $PATH, and you can then perform all kinds of tasks (like updating your website) without leaving your mail client.