Outlook 2003 sends the wrong userid

Can’t figure this one out! I have a computer that runs Outlook with 4 profiles.
When I test the accounts, the correct userids are used. However, when the user does a Send/Receive, the wrong userid is sent.
This is true for all of the other profiles too (except the one whose id is being sent…).
Done a complete re-install; even changed the desktop.
I have this same system running on approx. 150 desktops and it works fine. These four users are the only ones screwed up…!
Any ideas out there? Thanks for your time and replies!