I use the built in UCE (Anti Unsolicited commercial email) features of Postfix in conjunction with Spam-assassin and ClamAV
http://www.postfix.org/uce.html
http://www.postfix.org/CONTENT_INSPECTION_README.html
The more mail I can drop for obvious reasons on the front end of my MTA, the less junk the MTA or SpamAssassin has to process.. That's good for keeping my CPU and RAM usage lower on the mail server...
You could provide more info on your setup.. it might trigger more responses that are geared towards your config..