Outlook 2000 error: The operation failed

I’m running Outlook 2000 on a Windows XP pro IBM NetVista and connecting to Exchange 5.5 on SBS 4.5. Problem is Outlook has never been able to open users’ mailboxes. I’ve done this successfully on other boxes running the same mix of software but thethe five IBM units have consistently failed. I’ve tried every plausible suggestion from Microsoft’s Kb but nothing has helped so far. I’d be grateful if someone gave me a hand or hint.