Move mail boxes
If you move a user between servers, you only need to perform the handheldcleanup.
If you move a user within a server (different mailbox database) you need to either:
1) Temporarily assign that user to a static mailbox agent
or
2) Restart the server
KB00142 - Cannot send or receive email messages because a mailbox has been moved
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Restarts of a server are KNOT required when moving users from one store to another. When doing a mass move however it does make things easier.