Outlook web Access

I have a user that wants to check his mail using Web Access. We are running Exchange 5.5/SP2. He is an exisitng user and gets his e-mail (internal and extrenal) just fine. When I have him sign in we recieve the sign-in username/password screen three times then we are told we do not have rights to that mailbox. Other users in the same domain and exhchange server can access their mail thru the web without any problems. What I am doing wrong? IIS and Exhange are on the same server.