But please indulge me --- exactly what is the benefit of deprecating
the "A" fallback, and/or not doing a lookup on the AAAA record if the
MX record doesn't exist?
Under the current setup, any domain that has an A record is presumed
to be a mail domain, and if it's not, there's no good way to advertise
that fact. (Current crocks include running a stub server that rejects
everything, and the MX 0 . proposal.) In a world where the fraction
of mail that is spam is 95% and growing, the load from unwanted mail
to non-mail domains is significant. I have at least one host name
that was never a mail domain, but since it used to appear in usenet
headers it gets over 30,000 spams a day, every day.
As Keith recently pointed out, since it is no longer 1983, the vast
majority of names with A records are not mail servers nor are they
domains that occur in mail addresses. They're workstations or PCs
whose users get their mail through a domain and a server somewhere
else. Or with IPv6, they'll be toasters and doorbells, even less
likely to be mail servers.
Getting rid of the AAAA fallback flips the default to be in line with
reality -- most hosts don't want to receive mail directly, so if
you're one of the minority that actually does, you affirmatively
publish an MX to say so.
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