Unauthorized attempting to send Email with valid access token

OK, I've figured out the issue - putting this here for others to find if they hit the same problem.

The underlying issue is that the O365 v2.0 Web API is woefully under-documented!

The main issue with using the Client Credentialworflow is that it authenticates the App itself, and not a user, therefore you need to specify the user to send the email from in the URI - but I had to guess that from the v1.0 API!

Changing the URI used with the access token from ;

https://outlook.office365.com/api/v2.0/me/sendmail

to

https://outlook.office365.com/api/v2.0/users/<user spn>/sendmail

Then the API knows which user the email is being sent from and the token which has the 'Send mail for any user' claim is accepted and the email is sent.

Hope others find this useful.

Regards,

Mark.