Exim is a message transfer agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the Internet.
We find it highly useable, flexible and much easier to get your head round than Sendmail.
It is freely available under the terms of the GNU General Public Licence.
Useful Services
Spamhaus tracks the Internet's Spammers, Spam Gangs and Spam Services, provides dependable realtime anti-spam protection for Internet networks, and
works with Law Enforcement to identify and pursue spammers worldwide.
Spamhaus also tracks illegal 3rd party exploits, including proxies, worms and trojan exploits.