code: def log(self, message, result_code, log_message=""):
          
          return self.create(
              message_data=message.message_data,
              message_id=get_message_id(message.email),
              when_added=message.when_added,
              priority=message.priority,
              # @@@ other fields from Message
              result=result_code,
              log_message=log_message,
          )
docs: create a log entry for an attempt to send the given message and
          record the given result and (optionally) a log message