This behavior is by design. In Microsoft Exchange Server 2007 or in Exchange Online in Office 365, moving a mailbox between servers causes the mailbox to be locked. Therefore, the mailbox is inaccessible during the move process. The length of time for the migration to complete depends on the size of the mailbox.
Note�If a user is being moved to an Exchange 2010 server, they should have access to their mailbox throughout the move. They may be prompted to restart their Outlook profile as soon as the move is completed. However, the user should be able to access their mailbox immediately.
There are two reasons that a managed mailbox may be moved to another server so that the Exchange environment remains stable:
- Load balancing: In some instances a server database may have too many or too few mailboxes that are provisioned to it. To keep the services stable, some mailboxes may be migrated between databases or Exchange servers.
- Environment expansions: As users are added to the service, it is important to add new servers. Mailboxes may be moved to the new servers to relieve the load from other servers.
Mailbox moves are usually scheduled outside the usual business hours. However, in some instances you may have to move mailboxes at other times. If a mailbox is very large, moving it can take�additional time to complete.