I am of course recommending FC with Sendmail, Procmail, SpamAssasin and ClamAV on an inhouse mailserver, all of which I've had experience and spectacular results with.
Consider adding MIMEDefang (http://mimedefang.org/) to that mix. It runs SA and Clam from a milter invoked by sendmail, so you can detect the viruses and spam and reject them while you still have the sending MTA connected.
