Becoming a "Spamassasin"

I integrated and adminsiter a postfix / sa / clamd / courier system for about 10,000 domains (6 identical blades with local storage for the queues and Global File System GFS once they've been accepted).

SA gets about 95% accuracy on our system even at 5.0

I can also recommend enabling SPF and DKIM see http://www.openspf.org/ Wave bye bye to emails pretending to be sent from yourself to yourself or those annoying "your settings have changed, open this zip file" mails.