Corrupted “View by Sender”

We have a have a user using Outlook 2000, who can
only see a couple of messages when sorting mail by “Sender”. For instance, it will show that he has 7 messages from ‘Joe Blow’, but he can only see one or two. When he views messages in any other way (by date, received, etc)Outlook shows all the mail correctly. This user was recently migrated from Exchange 2000 to 2003. I have seen reports of this type of thing before, but can not find an answer. Any help is appreciated.