I have a real doozy here. I am no Exchange guru by any means, and have never seen this problem.
One of my clients has been trying to reach me. She sends a new email, no attachments, nothing special, to me, but I never get it. She does get other emails, she can send to other without issue too. I never get her email.
If I email her, and she replies, I get the email fine. Just new emails to me and I suspect others are not going out.