"Despite the the passage of the CAN-SPAM Act, email users are
still subjected to vast quantities of spam and virus-laden
messages. Roaring Penguin's CanIt-PRO is a Sendmail-based
application that helps block spam, viruses, phishing attempts, and
other nastiness. I found it to be one of the better commercial
Linux email filtering packages.
"Roaring Penguin makes packages for many Linux distributions,
and also provides source code--so it should be possible to run
CanIt-PRO on just about any Linux distribution. I installed the
product on a CentOS 4 system, using the packages for Red Hat
Enterprise Linux (RHEL) 4..."