Adds new signal for emails that cannot be processed - `email_received_unacceptable`.
This allows client applications who are subscribed to the signal to handle the inbound error. Errors are handled in this slightly unconventional manner as most external mail services require that the app return a 200 OK HTTP response, even if they can't handle the email, to prevent the service from endlessly retrying.
This approach allows the app to support both (conflicting) requirements - returning a 200 to the calling service, whilst still handling the error internally.