Graylisting is an idea invented by Evan Harris in 2003. Basically, the first time someone tries to send you mail, you send back a temporary failure response. A real email system will put the mail into a queue and re-try it later, at which time the address will have graduated from the graylist to the whitelist, so the mail will get through. However, spam engines and worms do not have real email systems. They are optimized for sending out mass quantities of mail, and they generally do not implement re-trying. Therefore mail from those sources will not get through.
The advantages of graylisting are that it's very simple, it requires zero maintenance, and it will never block any real mail. The disadvantage is that it delays mail. However, people you correspond with every day will stay in the whitelist and will get through immediately. For other folks, well, the delay is brief. There's more info about graylisting at greylisting.org.
Graymilter is a sendmail milter implementing a somewhat simplified version of the original graylisting concept. It's very effective and uses little CPU time. On a busy site like mine it does use a fair amount of memory to store the lists - one or two hundred megabytes. It's definitely worth it, tho.
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