Lost e-mail in Exchange

Ther eis one particular address coming into our mail system that “gets lost in transit” somewhere. Our spam filter says its being passed through, and Exchange system manager (Exchange 2003) says its being delivered to the correct mailbox, but the end user can not see these messages in their inbox (Outlook 2000 Exchange connection). This is the only address we are having problems with. Anyone have any ideas on where to look? the outlook client is not putting them in junk mail and has no filters configured.