Probably a simple problem with Outlook

Ive got a couple of users having some communication problems. One of the users can email everyone alse in the domain successfully. However, they are unable to recieve any emails form the second user. The second user can successfully recive from everyone, including user1. But user2 can send to everyone but user1. The messages just dissapeer without a trace. They seem to be sent successfully.
Were running Outlook 2000 and our email server is MS Exchange 5.5 running on NT 4.0 Small Business.
Thanks