You could also try inflex
http://www.pldaniels.com/inflex/about.html
It's not that difficult to configure, integrates w/ most virus scanners (we use MacAfee mainly because w/ needed Mac+PC+Linux support), and can be configured to do content based filtering. We do our spam checking at the sendmail level, before the virus scanner ever sees the message.
It pretty much uses sendmail, but could probably be modified to run w/ other MTAs.