We’ve tried with both OAuth and Credentials. Using Office 365. We have had success with other office clients but seem to be hitting a wall here with some others. We can retrieve the AccessToken with no issues, but get the above message with no additional information.
To reproduce this issue at our end, we need sample test Visual Studio application.
It would be great if you please share the SMTP activity log. You can generate it by following the steps mentioned here:
SmtpClient Activity Logging
Thanks for your cooperation and understanding.