We understand that many customers continue to face real challenges modernizing legacy email workflows and need sufficient time to adopt viable, secure alternatives. Based on customer feedback and vis...
Updated Jan 29, 2026
Version 3.0
Blog Post
1 MIN READ
Updated Exchange Online SMTP AUTH Basic Authentication Deprecation Timeline
ok, Got It.
How ever If need a way to SMTP the Basic Auth isn't work and the OAuth 2.0 face some inconvenience.
After following Microsoft's official documentation step-by-step to configure SMTP AUTH with OAuth 2.0 Client Credentials Flow (Application permissions) for sending emails from a third-party application (Microsoft Dynamics 365 Business Central), authentication fails with 535 5.7.3 Authentication unsuccessful even though all prerequisites are confirmed in place:
- Token is successfully issued by Microsoft Entra ID with the correct scope
- All required permissions are granted with admin consent
- Service Principal exists in Exchange Online with correct role assignments
- Test-ServicePrincipalAuthorization returns InScope: True for the target mailbox
- SMTP AUTH is enabled at the mailbox level
- More than 44 hours have elapsed since the last configuration change (replication should be complete)