This is happening because Microsoft is disabling SMTP authentication, which leads to the "5.7.139 Authentication unsuccessful" error that you see.
To fix this problem, you can try one of the following:
- (easiest) Tick 'Use automatic settings' in WiFi Guard settings. In this case WiFi Guard will be instead using SendGrid for sending mail.
- Use a different SMTP server.
- Re-enable SMTP.
To re-enable SMTP, you can try the following steps:
- Login to Azure Active Directory, select Properties in Manage menu, scroll down to "Manage Security Defaults", and disable security defaults by setting the slider to "No".
- Login to Microsoft 365 Admin Center and go to Settings - Org Settings.
- In Services, select Modern Authentication and make sure that "Authenticated SMTP" is checked.
- Then go to Users - Active Users, select the user from the list, then open Mail tab, click "Manage email apps" and make sure that "Authenticated SMTP" is checked for that user.