Outlook and OWA

Any ideas on why a particular client is not able to access mail from certain address’s and not others coming from internally? We can access the user’s mailbox from OWA, and the messages appear, but not from Outlook. We did transfer a new machine to the user, setup Outlook 2000 on XP, can still receive messages from most everyone, internally, and externally. We have established that they had rules setup to send emails from certain users to certain personal folders, and those rules seem to be some of the internal users that email isnt coming from.
Thanks