I just want to share a revelation with everyone.
I used to get lots of spam on my e-mail address that I serve at home. By lots, I mean around 150 per day. I was quick to turn on SpamAssassin, which along with procmail could automatically sort the spam away before I saw them, but even then, I got 10-20 spams that SpamAssassin couldn't detect through to my Inbox. By then, I had set up an automatic system to feed spams back into SpamAssassin's Bayesian learner via the mail system and all sorts of things.
When I went on vacation in late March for seven days, I came home to an Inbox filled with around 150 spam messages that I had to filter through. Needless to say, that wasn't too fun...
After that, I implemented a trick that I had heard of, Greylisting, since I saw a link to an actual implementation on Slashdot, and this, as you may have guessed, is the revelation of which I am speaking.
Greylisting is so great that I cannot praise it enough. I get no spams anymore. And by that I mean literally none. It's been weeks since I last saw a spam. Sure, there are a couple each that get through the greylisting, but SpamAssassin is still running beyond there, waiting for them. It catches around 5 spams per day these days.
Of course, I don't know how long it will take until spammers out-smart greylisting, but for now, it's just great. I recommend milter-greylist to anyone with even the slightest spam problem.