Hello Lukas,
the application already has full_access_as_app and Mail.Send (it is part of Office 365 Exchange Online) permissions: you can check it on following picture
https://drive.google.com/file/d/1EueLe1Z7XpVqWvKikPFOzB_ThzlQnEhJ/view?usp=sharing
The problem is this:
- if "_ews.Settings.Impersonation" and "Sender/From" of the message are the same email it works
- if "_ews.Settings.Impersonation" and "Sender/From" of the message are different I get the error
"The user account which was used to submit this request does not have the right to send mail on behalf of the specified sending account. Cannot submit message (ErrorSendAsDenied)."
Thanks