/bin/mail as MTA

 

/bin/mail as an MTA

Published 2015-12-29

If one looks at the manpage for the humble mail program (man 1 mail), it has a ton of options. I've used it over the years in hacky scripts and to read root's mail on some poorly configured server, but I never realized that mail can operate as an MTA (Mail Transfer Agent).

I was recently working on a configuration for redundant Postfix mail servers and Dovecot IMAP servers and needed a means to test. Looking at the manpage for mail, they give an example command, which I've modified and included here:

echo "Test email" | \
    env LC_ALL=C MAILRC=/dev/null password=${PASSWORD} user=${USER} \
    mail -n \
    -Sv15-compat \
    -Ssendwait \
    -Snosave \
    -Sssl-verify=warn \
    -Ssmtp-auth=plain \
    -Ssmtp-use-starttls \
    -S "smtp=submission://${MAIL_SERVER}" \
    -S "from=${SENDER_EMAIL}" \
    -s "Testing" \
    -. \
    $RECIPIENT_EMAIL

I'm sending "Test email" as the body of the message to $RECIPIENT_EMAIL from $SENDER_EMAIL with "Subject: Testing". I thought I'd explain the options in this post.

  • LC_ALL=C – no UTF-8 needed for testing.
  • MAILRC=/dev/null – don't read any local .mailrc to completely control the options used.
  • user…= and password…= – you can set the auth credentials in the smtp= URL below, but they need to be URL-encoded, so I took the lazy path and stuck them in the environment for everyone to see.
  • -n – don't read /etc/mail.rc, again to completely control the options used.
  • -Sv15-compat – use the newer options to mail.
  • -Ssendwait – the manpage indicates error reporting will be more reliable with this option set. I turned-off encryption and sniffed the wire to determine if there were any obvious differences, but I didn't see any. Needs more investigation…
  • -Snosave – no dead.letter files scattered everywhere!
  • -Sssl-verify=warn – I'm using a self-signed X509 certificate, so I need this option to negate cert verification. With Let's Encrypt, the days of bad certificate discipline may finally be over.
  • -Ssmtp-auth=plain – I'm using the plain authentication method.
  • -Ssmtp-use-starttls – for the submission: schema, you need this option or you won't be getting an encrypted session. This is not needed for the smtps: schema since that one implies encryption.
  • -S smtp=submission://${MAIL_SERVER} – the URL for the destination MTA. The manpage has the various schemas accepted, and you can specify the port if your not using the schema's default.
  • -S from=${SEND_EMAIL} – sets the envelope "from" field.
  • -. – indicates the end of options. I wonder why they didn't use the standard --?

Just thought this was neat.