I dont know if you have your own smtp-server?
If so, I can recomend you amavisd-new with spamassassin and clamd(and maybe some other virus-engine).

I have installed it in several companys and the results are really _great_! No viruses and almost no spam...

I'm in the works for a forwarding server for mail, that cleans it before retrivial. If youre interrested I can add you to it when I get there...

/Fredrik