This program is an implementation of a greylist policy service for
postfix (2.1.0 and later).  Greylisting is a concept to reduce the
amount of UCE ('spam') by technical means.  Tests on production
systems show that greylisting is hugely effective.

Read more about greylisting on:
WWW: http://www.greylisting.org and
WWW: http://projects.puremagic.com/greylisting/whitepaper.html

Also, please consult homepage of this program:
WWW: http://mimo.gn.apc.org/gps/