'L{MXCalculator.getMX} returns the MX record with the lowest preference
if all the MX records in the response have been marked bad.'

def test_fallbackForAllBadExchanges(self):

    domain = 'example.com'
    bad = 'bad.example.com'
    worse = 'worse.example.com'
    records = [RRHeader(name=domain, type=Record_MX.TYPE, payload=Record_MX(0, bad)), RRHeader(name=domain, type=Record_MX.TYPE, payload=Record_MX(1, worse))]
    self.mx.markBad(bad)
    self.mx.markBad(worse)
    return self._exchangeTest(domain, records, bad)
