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Microsoft 365 backups display the following error: The HTTP request was forbidden with client authentication scheme 'Anonymous'. This error is typically occurs when certain mailboxes are accessed during backup jobs.
After a coordinated investigation with Microsoft, we have confirmed that some subscription plans, such as Exchange Online Kiosk and similar limited-service SKUs, may not support the authentication mechanisms required by Veeam Backup for Microsoft 365. As a result, backup jobs fail because Veeam cannot authenticate against Microsoft’s endpoints, returning an HTTP 403 Forbidden error.
To resolve this issue, ensure that a supported Microsoft 365 plan that includes full Exchange Online and Graph API support is used (e.g., Microsoft 365 Business Standard, E3/E5). Plans should be verified against Veeam system requirements and Microsoft’s service descriptions: Veeam Backup for Microsoft 365 System Requirements Veeam Backup for Microsoft 365 Used Ports Veeam Data Cloud for Microsoft 365 Microsoft Entra Application Permissions Exchange Online Service Description – Features Available to Some Plans Office 365 Plan Options – Service Families and Plans
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