I had posted this in my old weblog before The Big Crash, but I think it needs an encore. There is obviously no single silver bullet in the spam problem, but I've found one technique that's been very effective at reducing it on my own webserver, with a guaranteed zero false positive rate. It relies on spammers being stupid.
The short description is that when our mailserver receives an inbound SMTP connection and the other end uses our own IP address in the HELO message, we reject the message out of hand. No real MTA has ever done this, so it's guaranteed that this is spam (and usually from Trojaned machines).
Unixwiz.net Tech Tip: Blocking spammers with Postfix HELO controls
It's turned away more than 18,000 connections just in the last week.
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