"Building a complete email system with spam and antivirus
protection is not as hard as you might think. This guide will walk
you through installing and configuring everything you need for
sending and receiving email, filtering spam, and scanning for
viruses in email.
"For our system, we'll use the Postfix mail transport agent
(MTA); Dovecot, a secure, open source IMAP and POP3 server for
Linux/Unix-like systems; SquirrelMail, a standards-based Webmail
package written in PHP 4; SpamAssassin, a powerful open source spam
filter; and ClamAV, a GPLed virus scanner..."