The Exim Mail Transfer Agent, a Replacement for sendmail

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.
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.