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.
I think it's vanishing unlikely that email will be useful at all, if spam
filter writers keep trashing mail based on such dubious criteria.
Not to reignite the usual spam argument, but it is (unfortunately in this
case) not 1988 or even 1998 any more. When upwards of 90% of all mail is
spam, keeping mail usable is at least as dependent on limiting the spam
that shows up in people's mailboxes as delivering the trickle of good
Real life spam filters use metrics and tune to the actual behavior of
mailers. If most of the mail that comes from domains with AAAA and no MX
is spam, they'll tune for that, and it won't be a mistake. For the
forseeable future, most such mail will be from zombies, and it'll all be
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