1650 - Exchange backup scan completes with error “This mailbox is not a valid Microsoft 365 mailbox”

Symptom

The scanning task during an Exchange backup completes with the following error:

You may see the following in the Storage Platform Console: "Backup scan failed: REST API is not yet supported for this mailbox".

Cause

This error can occur for several reasons.

• The mailbox in question does not have a valid Microsoft 365 licence, or has a valid licence that is of a type we do not support. For a list of supported licence types, see Article 1421.
• The mailbox is not part of an M365 plan that includes Exchange Online.
• The mailbox is an Outlook personal account that has not been enabled.
• The mailbox no longer exists.
• The mailbox is on a dedicated Exchange Server.
• The mailbox is hosted on-premises (Exchange hybrid setup).

Solution

The solution to this error will depend on the cause. To prevent the error, take the steps below as applicable.

1. In the Entra ID admin center, verify that the affected mailbox has an active and valid Exchange Online licence.

• If the mailbox is unlicensed, assign it a licence or remove the mailbox from the backup selection in the RedApp.
• If the mailbox is a shared mailbox, check that it is correctly configured in Entra ID.
  • A shared mailbox will typically not have a licence if it is under 50GB in size, but it may still appear in the backup selection, as explained in Article 1586. If a shared mailbox is causing the error, remove it from the backup selection or (if using group-based backup selection) remove it from the Microsoft Entra ID group that is selected for backup.
• If the mailbox has been soft-deleted, restore it or remove it from the backup selection.
• If the mailbox is hosted on-premises (Exchange hybrid), remove it from the backup selection.

2. Re-run the backup after making any changes. If the error persists, log a ticket with our support team.