John C Klensin wrote:
Whatever I said, I didn't intend it to convey the expectation
that the responsibility would need to lie with the IPv4-only
recipient site. I have come to believe that it is appropriate
for a receiving site to do at least superficial verification of
the possibility of delivering an NDN before accepting a message
for delivery for which an NDN might be necessary (i.e., for
which delivery cannot be assured while the SMTP connection is
still open). I think that testing the reverse path and making a
decision to not accept the message if the test fails is entirely
consistent with the "take responsibility" language of 2821 (and
1123).
Well I'll be. I must be still dreaming! PITCH ME! :-)
I think that implies that, for the near future at least, if
test(_at_)ipv6(_dot_)l(_dot_)google(_dot_)com wants to have a reasonable
expectation
that mail it sends will be accepted by mail servers running in
IPv4-only environments, then it (the sender) must expect to
either be dual-stack (and advertise the IPv4 address too) or to
have a lower-priority MX advertised that will accept IPv4
traffic.
Thank you!
Consider if that reasonable expectation is not met, we will be breeding
a new environment of send only systems, including the exploitation of
such new send-only behavior. Legitimate Ipv6 systems would not only
risk losing response/feedback communications, but it will put itself at
greater harm by getting itself blacklisted or blocked.
I can not phantom how we would allow such a new pattern of exploitable
behavior to persist and grow simply because we relaxed the methodology
we had in IPv4 based SMTP systems in the name of one way only
communications IPv6 system.
--
Sincerely
Hector Santos, CTO
http://www.santronics.com
http://santronics.blogspot.com