Outlook bombs when conecting to Exchange

I’m running Exchange 2000 on a Windows Server 2000 Domain Controller(SBS). Everytime I connect a new user to Exchange it works first time. The very next time I logon and open Outlook, click send and recieve, Outlook has encountered a fatal error and bombs out.
When I go into my Exchange settings for Outlook and try to remove user and add another user it bombs out too, this time with no errors.
Outlook version is XP PRO.
If I setup the same user on Outlook on the Exchange Box it works fine.
This problem persists with current AD users aswell as new users I create.
Any help is greatly appreciated as I am seriously stumped.