I have an organization with a single domain and 2 exchange servers-- we are migrating from Exchange 2003 to 2010, so both are currently in production. I have a gateway server that drops mail directly to my Exchange 2003 (by IP address), and then mail is scanned by Trend Micro's Worry Free Advanced (v8) and GFI Essentials 2012, and I believe the scan order is Trend first, then GFI. The mail is (then?) delivered to the correct mail server. I say this with trepidation because I may be unclear on the order.
The problem is this: some outside email from select people never makes it to the users, and don't generate bouncebacks/NDRs. I've done reputation checks on their mail servers and IP's for blacklist/rbl health checks and they are clean. Quarantine search on both Trend and GFI show nothing in the logs, but I can see from my gateway (3rd party) server that the handoff to Exchange on these messages succesfully complete no differently than others.
Users not receiving email aren't specific to one server: some are on Exchange 2010, some on Exchange 2003. I can see it arriving on the 2003 server in the smtpsvc log, and if I go to the message tracker in either server, no results.
I am looking for ideas on identifying the problem, and/or specific log file locations to walk/review.