Send LMTP QUIT command, read the server response and disconnect.

def quit(self):
        """
        Send LMTP QUIT command, read the server response and disconnect.

        """
        self._send('QUIT\r\n')
        resp = self._read()
        if not resp.startswith('221'):
            logger.warning('Unexpected server response at QUIT: ' + resp)
        self._socket.close()
        self._socket = None
        self._recipients = []
        self.results = {}