code: def enqueue(self, queue_identifier: QueueIdentifier, message: Message):
          
          assert queue_identifier.recipient == self.receiver
          with self._lock:
              already_queued = any(
                  queue_identifier == data.queue_identifier and message == data.message
                  for data in self._message_queue
              )
              if already_queued:
                  self.log.warning(
                      'Message already in queue - ignoring',
                      receiver=pex(self.receiver),
                      queue=queue_identifier,
                      message=message,
                  )
                  return
              timeout_generator = udp_utils.timeout_exponential_backoff(
                  self.transport._config['retries_before_backoff'],
                  self.transport._config['retry_interval'],
                  self.transport._config['retry_interval'] * 10,
              )
              expiration_generator = self._expiration_generator(timeout_generator)
              self._message_queue.append(_RetryQueue._MessageData(
                  queue_identifier=queue_identifier,
                  message=message,
                  text=json.dumps(message.to_dict()),
                  expiration_generator=expiration_generator,
              ))
          self.notify()
docs: Enqueue a message to be sent, and notify main loop