I was planning to install spamassassin+amavisd+clamav on our mail server for a long time, but every time I try to do it, I quit after few tries.
But finally I got it, and it isn’t that hard at all, although I don’t understand everything yet, but it seems easy. Hopefully I’ll add some configuration hints for FreeBSD of course.

• Spamassassin is SPAM mail detection software, it became the de facto standard in SPAM fighting.
• ClamAV is an opensource antivirus program.
• amavisd-new is a high-performance interface between mailer (MTA) and content checkers: virus scanners, and/or SpamAssassin.