Greylisting spam
I've been reluctant to implement greylisting as a form of spam prevention on my server. It basically works by rejecting all mail with a temporary "I can't handle this right now, resend later" error. The mail system is designed to be robust and this is perfectly acceptable... senders are required to deal with it by the the standard that defines how mail works. But spammers don't bother. So if you just say, "Resend later," the theory is, spam disappears.
We use it at work, and I have encountered annoyances with it before, when I'm on the phone with tech support and they say, "Let me email you a new license key" and I can't receive it while I'm talking to them because their server only retries every twenty minutes. I receive so little spam at work that I've put myself in the whitelist so that my mail doesn't get greylisted.
But at home I've gotten tired of updating my personal spam filter rules every day, skimming a couple _hundred_ subjects in my spam folder for false-positives every day. (I've had the same email address for over ten years, and I've never kept it hidden... every spammer in the world knows me.) So I decided to try implementing greylisting just on my personal account.
My spam count has dropped from a couple hundred a day to between 20 and 30 a day. And I've only had one or two slip past SpamAssassin in the last week, instead of ten a day.
I don't know how long it will last... the spammers can get around it pretty easily, though at a cost of speed, as they'll have to send every mail twice. But for now, it's working better than I ever dreamed it would.