In short, a method for reducing spam at the MTA (mail server) outlined at http://projects.puremagic.com/greylisting/whitepaper.html .
From the whitepaper:
"Greylisting has been designed from the start to satisfy certain criteria:
User-level spam blocking, while somewhat effective has a few key drawbacks that make its use in the continuing spam war undesirable. A few of these are:
As a result, Greylisting is designed to be implemented at the MTA level, where we can cause the spammers the most amount of grief."