By default, Enterprise Vault archives only the mailboxes that are listed in the Exchange Global Address List. In the Enterprise Vault Administration Console, hidden mailboxes are not listed in the Enable Mailbox wizard and do not appear in Exchange Mailbox archiving task reports. If you want Enterprise Vault to archive hidden mailboxes, you can use the registry setting, ProcessHiddenMailboxes.
Enterprise Vault does not process the mailboxes of disabled Active Directory accounts. Even if you have configured the registry value ProcessHiddenMailboxes, the mailboxes of hidden disabled accounts are not processed. To force Enterprise Vault to include the mailboxes of users whose accounts have been disabled, configure both of the registry settings, ProcessHiddenMailboxes and ExcludeDisabledADAccounts.
These settings are documented in the Registry Values manual.
To check if a mailbox is hidden