Douglas Otis wrote (2007-07-25 on the DKIM list):
At this point in time, it should be rather rare for incoming SMTP
servers to depended upon a AAAA record for locating their servers.
The DKIM WG should push to have A or AAAA record discovery
deprecated. Deprecating address record discovery techniques will
eventually simplify where policy needs to be published. At some
point in the future, not publishing an MX record for the originating
domain might cause a message to be rejected.
Hi, scanning old messages I saw that you said this more than once on
the DKIM mailing list. I'm also aware that Meng Weng Wong and others
proposed something in this direction on the SPF and MARID list back
in 2004. It's also related to the expired "null-MX" I-D, and because
of that it might affect various "NOMAIL" solutions (4408 "v=spf1 -all"
and Phil's I-D.hallambaker-nomail).
I'm not strictly against it, quite the contrary. *But* AFAIK it's not
planned to remove the "A fallback" from 2821bis, in fact 2821bis will
augment all discussions of A records with AAAA for IPv6 compatibility.
If you and others feel that the no-MX fallback should be limited to
IPv4 in 2821bis, as it arguably is in 2821, then please say so on the
SMTP list. Fixing the SMTP spec. for IPv6-only senders is something
between tricky and impossible, and your proposal could shift this task
from impossible towards tricky.
Frank
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