It seems there are two ways of looking at this:
(1) AAAA records in the IPv6 world should do exactly same
things as A records in the IPv4 world, so SMTP should look
for an AAAA record in the absence of an MX record, just as A
records are used in the absence of MX records.
(2) Although some SMTP servers will continue to be found
through A records for legacy reasons, there is no longer a
good reason for any new server not to have a published MX
record. SMTP clients (senders) will, of course, need to
continue to look up A records, but since there is currently
no significant use of AAAA records for email routing, we
should not perpetuate this legacy in IPv6 as it is in IPv4.
I agree with Jim's characterization and IMHO both positions
I also prefer (2) because I don't think the original "A
fallback" was meant to stay there very long and we just never
got around to deprecating that feature. If you ask a random
sampling of postmasters and DNS domain owners, I doubt many
would even remember right off the bat that such a fallback
Based on some small experience with email deployment and
operations, I believe that you are wrong.
I have to agree.
Indeed, if you asked
a random sampling of those groups --remembering that there are a
huge number of SMTP servers in the world, only a tiny fraction
of which are professional operations and with an even smaller
fraction being large-scale, carefully-managed production ones,
you might discover that many of them had forgotten that there
was such a thing as an MX record and how to set it up.
And even if they know MX records exist they may not be able to use them. Some
DNS provisioning arrangements allow users to set up MX records but there are
others that do not.
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