DISCLAIMER: There is no guarantee that I have any clue. Use
at your own risk.
Things I have cooked up for sendmail milters
A detail-less list of milters findable on the 'net
spamass-milter whose home page can be found at http://savannah.gnu.org/projects/spamass-milt/
Richard Gooch's mailutils milters whose home page can be found at http://www.safe-mbox.com/~rgooch/email/index.html