Recover a Mailbox
There are two ways to make an Exchange mailbox unusable: by deleting the mailbox with System Manager, or by deleting its user from Active Directory, which makes the mailbox unowned.
Deleted mailboxes can be recovered only by restoring them from a backup. Mailboxes belonging to users deleted from Active Directory can be recovered.
Note In very specific disaster recovery circumstances, you may need to remove Exchange attributes from an object before reconnecting the Exchange mailbox.
To recover a mailbox for a user deleted from Active Directory:
Administrative Group or Organization