Semi's Spam Milter (sspamm) is python based SPAM filter for Sendmail that utilizes spambayes heurestic spam filtering and virus scanner (BitDefender, not included). You also need Python Milter and Spambayes.
At my systems our sspamm filters 50K mail messages per day. Finds about 94% of spams, and passes only about 2% (that should be blocked).
Filter is created to be customisable as possible.