Depends on your skill level.
Greylisting (
http://projects.puremagic.com/greylisting/) -- this probably knocks out 80% of the spam right off the bat.
I'm not a fan of SpamAssassin, but, many people like it. A quick procmail or mailfilter recipe can get rid of tagged spam. Beware, dealing in the adult industry, you'll likely delete valid messages due to their pre-concieved notion that porn = spam.
My personal favorite is DSpam (
http://nuclearelephant.com/projects/dspam/) which is statistical and trainable. There is a quarantine section so that you can see the spams that were tagged and retrain if it has a false positive.
There is also CRM114 which is again statistical.
Another thread that discussed this earlier this year:
http://www.greenguysboard.com/board/...ad.php?t=16918