Mail Source - Microsoft Exchange Online not working

Make sure that you have not incidentally swapped the Tenant ID and Client ID. Happened to me :-)

Sadly this is not the case for me.

If you've double-checked the configuration values, you could try these steps next:

• Verify that TLS 1.2 is enabled on both M-Files Server and the computer where M-Files Admin is run, even though your error message is different than what's used as an example in this support article: Azure AD TLS 1.0 and 1.1 Error
• Enable mail source logging and see if these logs would contain any additional information: Collecting Logs for External Mail Source

In our case the problem was in Azure.  The Name of the Tennant had Spaces in it . We had it renamed to the primary domain name and then it worked.