Such a patch exists. Get it at http://homepage.cistron.nl/~miquels/nospam/
JC! Guru! Great minds think alike. :-) (I noted it in private email)
We use it, and have been using it for quite some time now. It only needs integration with the sendmail-8.8.8-cf suite, to call Parse0 to filter local crud before applying the anti-relay rules.
I actually just updated it for 8.8.8 (re-made the .diff and rewrote the .cf stuff so that it actually works!). Miquel van Smoorenburg has just updated his homepage. -- Niels Bakker, * * EuroNet Internet BV Network Operations * * Herengracht 208-214 * 1016 BS Amsterdam NJB9 * +31 (0)20 535 5555