I am on an internal course this week called Exchange Environmental Dependencies. Brian is doing an exellent job of teaching us (check out his blog http://reidablog.blogspot.com/)
Brian mentioned greylisting smtp conversation today. I have heard of it before, so I as usual I googled a bit and come up with this from http://www.windowsitpro.com/Article/ArticleID/95331/Windows_95331.html
Whitelists specify senders or connections that you always want to accept mail from, and blacklists (or block lists, as Microsoft calls them) are senders or connections from which you never want to accept mail. A greylist is in between these two extremes: It’s a list of senders or connections from which you might not want to accept mail.
Have a read, it is interesting!