Moving a mailbox in any version of Exchange is typically an easy and painless operation. But migrating a large mailbox from Exchange 5.5 to Exchange 2003 can be the exception.
As long as everything is set up and working properly, moving a small mailbox works as you would expect. However, moving larger mailboxes is a different story.
Everything goes well until the very end of the mailbox move, when the Exchange System Manager's Task Manager dialog box displays Saving Changes To The Directory in the State field. At this point, the state never changes, and it hangs forever.
But if you cancel the mailbox move at this point, the mailbox stays on the original Exchange 5.5 server. So, how do you successfully move a mailbox from Exchange 5.5 to Exchange 2003?
While there's no definitive answer, there is a workaround you can try. This involves turning off replication over the Active Directory Connectors (ADCs). Stop replicating information over the ADCs, and then try moving the mailbox again. Once the mailbox moves are complete, don't forget to turn on replication again.
Again, this workaround isn't fail-proof, but it's an option you can try.