Renaming account name

I have renamed one of the user account name in Domail Controller under NT4.0. Now the user can login with the new account name but he can’t see his mail. I am using MS Outlook with Exchange Server 5.5. I can see the user has got some new mail in hisInbox but in the browser no mail is displayed. I have try to renamed his account name to the original but still he can’t see his mail. Please assit me what is the problem and how I can solve it.