"Use Temporary Mailbox" or "Use Old Data" when opening Outlook

We had a problem with our information store last week.  (exchange server 2003)
While restoring the last working version from backup tape, we created a blank store, to allow people to send/receive. Later we merged the restored mailboxes with the new store.

However, now, when people open their Outlook, it still asks if they want to open the "new mailbox" or use the "old data". The old data is data that was cached on their local computer (even though we don't use cached mode in outlook).

How do I make this message disappear for once and for all, and have people use the "new mailbox" (which contains the newly created + merged restored mail)?
Every time you open Outlook, it asks again.

Thanks!