SpamAssassin is a mail filter which attempts to identify spam using text
analysis and several internet-based realtime blacklists.
Using its rule base, it uses a wide range of heuristic tests on mail
headers and body text to identify "spam", also known as unsolicited
Once identified, the mail can then be optionally tagged as spam for later
filtering using the user's own mail user-agent application.
In its most recent test, SpamAssassin differentiated between spam and
non-spam mail correctly in 99.94% of cases. Since then, it's just been
getting better and better! (taken from the homepage)
To install it:
cp /usr/pack/spamassassin-3.1.1-ds/procmail.example ~/.procmailrc
(note that new accounts have spamassassin configured per default)