Re:Force keywords in the subject line (Score 1)

Sure, getting all the clients to send all their emails with [companyname] in the subject line is a little annoying, and may not be possible depending on your circumstances, but the chances of spam having that keyword within it is virtually impossible.

A spear-phishing attack like we've been seeing for the past few months where I work would actually make that extremely likely, unfortunately. For the OP, I've had a great deal of success running a Postfix/SpamAssassin/ClamAV setup. Not very hard to setup, and does a great job.