ssmtp (extremely simple MTA to get mail off the system to a mail hub)

sSMTP, replaces sendmail on workstations that should send their mail
via the departmental mailhub from which they pick up their mail (via
pop, imap, rsmtp, pop_fetch, NFS... or the like). This program accepts
mail and sends it to the mailhub, optionally replacing the domain in
the From: line with a different one.

Normally, when using ssmtp, you want to remove the OS's MTA
package(s), sendmail and/or postfix. In this case, ssmtp will
be symlinked to /usr/sbin/sendmail, and software that sends mail
shouldn't have to be modified. If you keep sendmail/postfix installed,
there's no conflict, but any software that sends mail will have to be
configured or modified to use /usr/sbin/ssmtp.

NOTE: After installing, you'll want to edit "/etc/ssmtp/ssmtp.conf".
There's a man page for it (man ssmtp.conf).