Log Message:
Initial import of msmtp, an SMTP plugin for MUAs, provided by
Bryan Carter Vyhmeister in PR 22840.

and probably other MUAs (mail user agents).  msmtp forwards mails
to an SMTP server (for example at a free mail provider) which does
the delivery.

Features include:
    * SMTP AUTH methods PLAIN, LOGIN and CRAM-MD5
    * TLS encrypted connections
    * IPv6 support
    * robustness
    * detailed error messages (including the full answer of the
      SMTP server) if something goes wrong
    * sendmail compatible exit codes (which most MUAs understand).

Simply tell your MUA to call msmtp instead of /usr/sbin/sendmail.