Why are Exchange Tabs missing in AD?

I have an Exchange 2003 server running on a Windows 2000 AD. When I go into users and computers on the Exchange server not all of the users property pages have the Exchange tabs, from what I’ve seen so far about 90% have the Exchange tabs and the others are missing them. The ones that are missing them do show up under exchange admin as having mailboxes and are on the GAL. I’m not sure if these accounts were part of a migration from Exchange 2000. How can I fix this? Is there a way to tell if these accounts were migrated?