Exim is a mail 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 overall style it is
similar to Smail 3, but its facilities are more extensive. It contains
facilities for verifying incoming sender and recipient addresses, for refusing
mail from specified hosts, networks, or senders, and for controlling mail
relaying.

WWW: http://www.exim.org/
