Query the DNSBL service for given value.

        :param host_object: an object representing host, created by
        self._host_factory
        :returns: an instance of dns.resolver.Answer for given value if
        it is listed. Otherwise, it returns None.

def _query(self, host_object):
        """Query the DNSBL service for given value.

        :param host_object: an object representing host, created by
        self._host_factory
        :returns: an instance of dns.resolver.Answer for given value if
        it is listed. Otherwise, it returns None.
        """
        host_to_query = host_object.relative_domain
        query_name = host_to_query.derelativize(self._query_suffix)
        try:
            return query(query_name)
        except NXDOMAIN:
            return None