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Why Are My Veeam Backups Not Working With Microsoft 365 Mailboxes Right Now?
Many users are having problems with Veeam Backup for Microsoft 365 mailboxes since mid-November 2025. The backup does not work for some mailboxes and gives this error:
- “The HTTP request was forbidden with client authentication scheme ‘Anonymous'”
- Not every mailbox is broken—just some.
This started after Microsoft made a change for Microsoft 365 mailboxes. The issue appears when using Veeam Backup for Microsoft 365 version 8.1.2.3301 and different Veeam versions on various subscription plans.
What Causes This Veeam Mailbox Error?
- Microsoft changed how some mailbox accounts allow connections.
- Some Microsoft 365 plans (like Exchange Online Kiosk and similar basic plans) do not support the new security requirements.
- Veeam Backup needs proper permissions from Microsoft 365 to access mailboxes, but it now gets blocked by Microsoft and shows the HTTP 403 Forbidden error.
- Plans reported as affected include Microsoft 365 Business Standard, E3/E5, Standard, and Business Premium.
What Do You Need to Do?
- Check your Microsoft 365 plan. Make sure it is supported for Exchange Online and the Graph API (not all “Kiosk” or very basic mailboxes are).
- Read the Veeam support article, KB4796, for details on which plans work.
- Talk to your Microsoft 365 admin about upgrading mailbox subscriptions if backup keeps failing.
- Ask Veeam Support for help if you are not sure.
Key Points for Users
- Not all mailboxes break—only some, depending on their plans.
- The problem started after Nov. 18–19, 2025.
- Ensure your Microsoft 365 plan supports authentication for all mailboxes you want to back up.
- The solution may require switching affected mailboxes to a supported plan.
For Extra Help
- Join the Veeam Forum discussion to learn how others are fixing this issue.
- Read the latest support and help articles for updates.