I got an interesting spam: it was PGP signed.
Yep, spammers are finding out that software such as spamassassin give negative points (no spam) to such messages.
Of course, the checking of the PGP signature gave me an invalid result, but SpamAssassin don't know that.
The spam was caught by bogofilter, a bayesian filter.
And here we go! Several levels of anti-spam. There's been a tiny amount of false positives with bogofilter. I pratically miss one or two spams a day, in a universe of almost 200 spams.
This is a better ratio than when I used spamassassin alone. (SpamAssassin is deactivated now, so that I can see how effective bogofilter is by itself...)
BTW, I haven't tried the bayesian feature of spamassassin... It should give a similar result to bogofilter.
I'm really thinking on how to implement such tools at clients of mine in a way that they won't poison themselves and start missing messages.