The Retain Archive Address Book contains all users (current and past) who have archived data. These might or might not have associated Retain User Accounts through which they can access the Retain Archive and perform administrative tasks.
As long as users have archived messages, their mailboxes cannot be removed from the archive’s address book. For more information about removing users who no longer have archived messages, see Deleting Mailboxes
in the Retain 4.10: Configuration and Administration guide.