IPv4-only hosts can see the AAAA record even if they can't
directly send mail to that address. and there's no reason
("obvious" or otherwise) why a MTA should reject mail from
a host just because that MTA can't directly route to it
Well, other than the practical fact that it's almost certainly spam.
Not to be cynical or anything, but regardless of what we decree, I
think it's vanishingly unlikely that many systems on the public
Internet* will accept mail from a domain with only an AAAA record.
When I consider the theory that there will be interesting numbers of
mail systems that do all the stuff they need to do to upgrade from v4
to v6, but that their DNS will be hosed in a way that lets them add
AAAA records but not MX records, I'm sorry, but that doesn't pass the
At the current stage of 2821bis I agree that the best we can do is
to preserve the existing ambiguous language, but down the road we
will either have to align with reality or take a courageous stand
against it. I know which I'd do.
* - that's public Internet, not to be confused with all of the local
routing hacks we have within our own networks
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