For mailbox protection, mailboxes are associated with one default User Mailbox subclient.
You can associate individual mailboxes, mailboxes within a database, and mailboxes within an Active Directory group (AD group). You can add mailboxes to the subclient using any of those groupings.
If you add mailboxes by databases or by AD groups, it might take a long time to discover the mailboxes right away if your organization is very large. You have the option to autodiscover the mailboxes in the groups later when the domain is scanned and the mailboxes are discovered at that time. Typically, this occurs once every 24 hours. If you choose to autodiscover mailboxes later, the mailboxes do not appear in the mailboxes list until after they have been discovered.
Note: Autodiscovery is not supported for dynamic AD groups.
You can assign Exchange plans to the mailboxes at the time that you associate them with the subclient or later. We recommend that you assign a plan immediately so that your mailboxes are protected.