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)
Send a single message instance.

    If TESTING is True the message will not actually be sent.

    :param self: a Message instance.