Use the Microsoft 365 admin center to enable or disable SMTP AUTH on specific mailboxes
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Open the Microsoft 365 admin center and go to Users > Active users.
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Select the user, and in the flyout that appears, click Mail.
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In the Email apps section, click Manage email apps.
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Verify the Authenticated SMTP setting: unchecked = disabled, checked = enabled.
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When you're finished, click Save changes.
