I recently set up SBS 2011 for a client. Everything works except that e-mail from one particular customer is not being received. I've looked in the Tracking Log and no e-mails from that address appear. The domain ends in .US instead of .com and I'm wondering if that has something to do with it. I've looked at the receive connector, transport rules, etc. in Exchange and nothing looks like it would restrict mail from a certain domain.
I checked the sender's domain to make sure they weren't blacklisted. Their IP address came back clean. I added the IP address to the While List in the Exchange spam filter thinking it was being blocked as spam, but that didn't fix the problem.
If the sender copies me on his e-mail, I receive it through my POP account.