Use the Microsoft 365 admin center to enable or disable SMTP AUTH on specific mailboxes
- Open the Microsoft 365 admin center and go to Users > Active users.
- Select the user, and in the flyout that appears, click Mail.
- In the Email apps section, click Manage email apps.
- Verify the Authenticated SMTP setting: unchecked = disabled, checked = enabled.
- When you're finished, click Save changes
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*Created using https://learn.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/authenticated-client-smtp-submission