E-mail msg goes to OWA, not Outlook 2000

We have Exchange Server 5.5(SP3) and using Outlook 2000, and Outlook Web Access for the clients. A user is having a problem in which he is able to view certain e-mails using the web access, but those messages doesnt show up in Outlook 2000, althoughit reports that he has unread messages. So far it seems to be happening only on some external (smtp) messages, but not all of them. Any suggestions ?