How do you do your filtering?

We use proprietary software to examine the headers of the email. Most spam is easily recognizable by the ways that spammers abuse the system, and those are often detectable from the headers alone. But some spammers, either the really new ones that don't know how to abuse the system, or the really experienced ones who know how to hide it better, can still slip spam past header tests. We're working on more sophisticated software which will do some screening on the content of the message, to eliminate even those.    We also use the MAPS RBL, DUL, and RSS lists to screen out mail from suspicious sources.