I am running Exchange 2003 on a Windows 2003 box and we are having issues with some incoming mail not being delivered. We were using two forms of SPAM filtering, Postini and GFIMailEssentials. We went over our license for the GFI product stopped working, but now some incoming mail seems to no longer be delivered.
One example, we have a user with a Gmail account who when he sends mail to his exchange account it doesn't go through. There is no bounced message and nothing that I can see on our server. Also, no record of it hitting Postini. I can send items from my Gmail account and they come right though.