We've been experimenting with Exchange Server 2003's Recovery Storage Group feature, but the Exchange System Manager (ESM) Move Mailbox tool doesn't always work. Why not?
The Recovery Storage Group probably already has a copy of the target mailbox in it. The ESM Move Mailbox tool expects that the destination mailbox database doesn't already contain a mailbox for the user whose mail you're moving. If such a mailbox already exists, the tool has to stop. This behavior is logical given the way that Exchange moves mail: by copying all messages from the source to the destination, with no attempt at merging or checking for conflicts. For this very reason, most Microsoft documentation suggests that you use the Exchange Mailbox Merge (Exmerge) utility to move mail between production storage groups (SGs) and the Recovery Storage Group. Exmerge is explicitly designed to gracefully place into one mailbox a unified copy of all mail from two mailboxes.