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Error ‘535 5.7.3 Authentication unsuccessful’ when using OAuth2 with MS Office 365 to send e-mail


If you’ve setup OAuth2 authentication for sending with SMTP from your Microsoft Office 365 account and you receive this error:

535 5.7.3 Authentication unsuccessful [CWLP123CA0136.GBRP123.PROD.OUTLOOK.COM]

(535 5.7.3 Authentication unsuccessful)

This probably means that you need to enable ‘Authenticated SMTP’ in your Office 365 account.

Fix it by following these steps:

1. Sign into your Microsoft Office 365 account on https://www.office.com and click on ‘Admin’

Sign into office365 and click on 'Admin'

Sign into office365 and click on ‘Admin’

2. In the Microsoft 365 Admin Center click on ‘Users -> Active Users’

3. Next select the user that should have SMTP enabled and click on the ‘Mail’ tab.

4. Click on ‘Manage email apps’

Click on manage e-mail apps

Click on manage e-mail apps

5. In the next window make sure that ‘Authenticated SMTP’ is checked.

Enable authenticated SMTP

Enable authenticated SMTP

That should do it! You should now be able to send e-mail using OAuth2 on SMTP with Microsoft Office 365.