Extsmail masquerades as the standard UNIX sendmail program, reading messages, and later piping them to user-defined commands.

In a sense, extsmail can be thought of as a very simple "tiny" sendmail (Think SSMTP, UUCP, ...)

A typical use is to allow e-mail to be piped via ssh to external servers running a full sendmail-compatible MTA. extsmail is designed to have sensible defaults, and configuring it is a one-off, quick job.