def lookupMailExchange(self, name, timeout=None):
    d = defer.Deferred()
self._reactor.callLater(5, d.callback, [(dns.RRHeader(name, dns.MX, dns.IN,
    600, dns.Record_MX(1, 'mail.example.com', 600)),), (), ()])
DNS server using Python and Twisted - issues with asynchronous operation