here is my issue.
our accounting department with to send email from an account called "accounting" however no user/mailbox exists under that name, they can receive mail on that address but testing sending gets the error.
You can't send a message on behalf of this user unless you have permission to do so. Please make sure you're sending on behalf of the correct sender, or request the necessary permission
Now, since no actual person with the name exists, if there someway of sending from that account without having to create a user in AD / MAIL.