Effectiveness of Greylisting

I have now implemented Greylisting on many mail sites, from small, to the largest being the mailhosts for the University of Alberta (@ualberta.ca). In 2005 mail volume in terms of smtp sessions was reduced by 50 to 65%, in 2006 it is approaching 80%

In the three days before this conference there were roughly 3,000,000 connections to ualberta.ca mx records over a 72 hour period. During that time about 450,000 smtp sessions were processed by the mail servers, the rest blathered at the greylister and went away or got trapped.

This reduction can vary of course, depending on how big of a target your site is.

Common Call after implementing: "Mail is Broken!" (People get scared when they don't get spam)