docs: def _message_send(self, connection):
      """Send a single message instance.
  
      If TESTING is True the message will not actually be sent.
  
      :param self: a Message instance.
      """
      sender = current_app.config["MAIL_SENDER"]
      if not self.extra_headers:
          self.extra_headers = {}
      self.extra_headers["Sender"] = sender
      connection.send(self, sender)