This error generated by a workflow on create of a record is driving us mad!
The workflow that's trying to send the email is running as "the owner of the workflow," which is a user account with Administrator privileges. We have also logged in as that user and confirmed the User Options has "Allow other Dynamics Users to send email on your behalf."
We are running in a hybrid environment; i.e., Online 2016 with Exchange on prem.
What else could be generating this??