Outbound email, particularly through Office365/GMail and Exchange Web Services fails with this 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"
Email Account Name and credentials are OK and are working.
This error is due to the Email Address (the From: address) is incorrect for the mail server and the email domain you are trying to send through. The mail server is rejecting it.
You can find these email addresses (typically) in the Global Constants as shown below (although you can use just about any From: address you want in individual Quick Actions, Notification Blocks in workflows and Business Rules).
Make sure these From: Addresses are correct for your Domain and will be accepted by your Mail Server - this will require some coordination with your Mail Server admins if you find the ones you are using do not work.