I've recently deployed a new spam filter at work and I'm rather proud of the results after the first few weeks. This blog post ought to be useful to at least someone who gets here by way of a Google search at some point I hope.
The new server is setup as my company's MX and relays mail inside the network as appropriate, but obviously only after it's been accepted. One key part of the design is to check all mail for a valid destination, viruses, and spam likelihood before it's ever accepted for delivery.