At 12:56 -0400 on 04/04/2008, Tony Hansen wrote about Re: current
usage of AAAA implicit MX?:
These questions are intended to find out what current IPv6 deployments
do in these situations. When an MX is not found, are they already
looking for both A and AAAA records, or are they only looking for A
records. Are they following RFC 3974 or not? What's the installed base
already doing?
RFC 3974 talks about Dual Stack set-ups and what an IPv4-Only Server
should do when presented with a Mixed MX or an MX that only points at
IPv6 AAAA records. It, BTW, REQUIRES an MX if there are any IPv6 MTAs
and only supports A-Fallback in the absence of an MX (ie: It does NOT
define any attempt to look for AAAA records in the absence of an MX
after/before checking for A records).
It also states that in the presence of an MX containing IPv6 address
references, an IPv4-Only stack should ignore the IPv6 addresses and
only use the IPv4 addresses (and in a IPv6-Only MX to NOT do
IPv4-A-Record fallback).