<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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)