Frank Ellermann wrote:
Keith Moore wrote:
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
What I wrote was "at their border", that is not any MTA on a
route from sender to receiver. "At their border" is the hop
where receivers decide if they accept the mail, or reject it.
If they accept it, and later find they can't deliver it, and
it is not a case for /dev/null, they MUST report the problem:
you're assuming lots of conditions there that don't apply in the general
case... e.g. single-hop internal routing for inbound mail and yet the
MTA can't detect that the mail is nondeliverable until after it has
accepted it, and no designated relay to the IPv4 world for outbound
mail. the former is occasionally true, the latter would be insane. one
way to look at it is that the border for inbound mail may be different
than the border for outbound mail.
| it MUST construct an "undeliverable mail" notification
| message and send it to the originator of the undeliverable
| mail (as indicated by the reverse-path).
For IPv4-only back to IPv6-only that cannot work, therefore
the IPv4-only border MTA is obliged to reject mails with an
IPv6-only envelope sender address.
often, the inbound MTA for a domain lacks reliable knowledge of whether
either the sender or recipient address is actually valid. it appears
that you would have the inbound MTA drop mail based on a very dubious
presumption. you would rather cause additional delivery failures of
subject messages than to risk the failure of nondelivery reports. most
operators, I think, would have the opposite preference. and regardless
of what 2821 says, I don't think your reading of it is justified.
there's nothing that requires a user to use the same MTA
for outbound mail that is used to receive inbound mail.
My definition of IPv4-only is that there is no other route.
that's a pretty useless definition because the case would not exist in
practice until IPv6 were ubiquitous. there's always the possibility for
an IPv6-only domain to contract with an MTA sited on both v4 and v6 to
relay outgoing mail. it would be insane for an IPv6-only domain that
wanted to send and receive mail to not have such an arrangement.
The IPv4 border MTA needs to know if that is the case or not
for its decision to accept or reject IPv6-only envelope
sender addresses, because if all else fails it MUST [s.a.].
if we were to take your presumption to its logical conclusion, every
inbound MTA should just reject all mail - since it can never be 100%
certain of being able to either deliver the message or send an NDN.
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