* Stephan Lagerholm: >>There is no way to signal the absence of an AAAA RRset, so you still >>have to make another round-trip to the DNS resolver if you're >>IPv6-enabled and the MXes aren't. (A and AAAA RRsets aren't cached as >>an atomic unit.) > > Good point. I wonder how many MTAs that actually check that... Exim does. We learnt it the hard way, after seeing failures in the field.