extsmail enables the robust sending of e-mail to external commands. In
effect extsmail masquerades as the standard UNIX sendmail program, reading
messages, and later trying to send them by user-defined commands. A typical
use of extsmail is to allow users who regularly move between different
networks and / or find themselves regularly offline, to ensure that their
e-mail is sent reliably via ssh to external servers.

In addition to its main purpose, extsmail also allows priority lists to be
defined, and also for e-mails to be routed to different servers depending on
the e-mail content. The former feature allows one to designate a "preferred"
server to send e-mail, but to use a "backup" server if the preferred server
is down. The latter feature allows e-mails sent to different e-mail
addresses, for example, to be routed to different servers; this allows
e-mails to adhere to the IP addressed filtering increasingly used to cut
down on spam.