Please add GreyListing to the Messaging Gateway. We are getting slammed with the first wave of every new campaign that doesnt have rules yet and have implemented all that we can do but still get missed spam to employees. GreyListing has proved to be effective enough in the past where I would happy take the learning curve hit with initial rejections to get the good email ratio up to over 90%. There are features in other vendor solutions that use GreyListing with a learning period to even cut down on blocked emails at implementation.
Please consider GreyListing, as it's a feature I am strongly considering going with as a primary or even only source of spam filtering with my Exchange server during our next upgrade project.