Symptom

Exchange mailboxes that are unlicensed and are not shared mailboxes are shown to be backed up along with the regular backup selection.

Cause

An unlicensed mailbox is backed up for as long as Microsoft 365 makes it available. Microsoft can disable an unlicensed mailbox some time after the licence is removed. From that point, the backup scan for that mailbox fails with the error described in Article 1650.

Solution

If this is causing problems, you can deselect the unlicensed mailbox/es from the backup selection (if selecting by user), or remove the mailbox from the Entra ID group (if selecting by group) to exclude it from future backups.