i'm currently running exchange 2010. lately, some emails sent to some of our email boxes are not getting delivered. for example, an outside sender writes an email to user1, user2, user3. user1 and user3 will receive the email but user2 never gets it. there are no failures in message tracking. if that outside sender tries to send an email to JUST user2, message tracking doesn't acknowledge the existence of that email at all. it just doesn't show up. this happens to a few specific email boxes on our server and seems to happen with specific senders too.
how do i start to track down the problem?