Unable to receive email from certain user

I have a user that is unable to receive email from another user within the same domain. I am running a 2000 domain (native) with Exchange 2000. Exchanges tracking tool shows tha the message was successfully delivered to the users mailbox, but it still doesn’t show up in Outlook. He can receive email from everyone else just fine. The user trying to send the message can also send to all other users except this one. Any ideas?