"Exim is a message transfer 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 some defences against mail bombs and
unsolicited junk mail in the form of options for refusing messages
from particular hosts, networks, or senders. It can be installed in
place of sendmail, although the configuration of exim is quite
different to that of sendmail."
The paragraph above quotes from the Exim website at www.exim.org.
Exim is available from:
and from many mirror sites - a list is available on the Exim site.