Exim is a mail transport agent (MTA) developed at the University of
Cambridge for use on Unix systems connected to the Internet. It is
freely available under the terms of the GNU General Public Licence.
In style it is similar to Smail 3, but its facilities are more
extensive, and in particular it has options for verifying incoming
sender and recipient addresses, for refusing mail from specified
hosts, networks, or senders, and for controlling mail relaying.

Homepage:
https://www.exim.org/
