A. Exchange 2010 introduces a new online mailbox migration capability that's used when migrating from another Exchange 2010 mailbox server or an Exchange 2007 SP2 or above mailbox server. With an online mailbox migration, users being migrated can still access their mailboxes while they're being migrated, minimizing the end user impact.

The alternative is an offline mailbox migration, which means users can't access their mailboxes while they're being migrated. This usually means the mailboxes need to be migrated outside of normal business hours.

If you need to migrate a mailbox back to Exchange 2007 from Exchange 2010, it will have to be an offline mailbox migration.

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