Highly Recommended Solution (Score 1)

I've built this: http://www.gentoo.org/doc/en/mailfilter-guide.xml, when i was tasked with setting up mailfilter. It works great and is also scalable with some LVS if you really need it. There's always GMail for business (former Postini) mailfilter (which is pretty cheap) or a ton of similar solutions out there that will do it for you.