Filter outbound mail?

[snarkout]

I need a good way to filter outbound mail -- we have inbound covered, but outbound is (so far) a total PITA. I'm using sendmail, and I'd really like to find a good way (read very few false positives) to flag outgoing messages as spam and/or coming from an infected computer (well, if they are). I'm looking at milter, but to be honest, I don't really understand it. Anyone have pointers or good sources for reading?