Bill Manning wrote:
example.com. soa (
mailhost aaaa fe80::21a:92ff:fe99:2ab1
is what i am using today.
In that case adding an MX record pointing to mailhost
or not is perfectly irrelevant from an IPv4-only POV:
IPv4-only users cannot reach your AAAA, therefore they
better reject mails claiming to be from any(_at_)example(_dot_)com
at their border for obvious reasons.
It also breaks the broken 1123 5.3.6(a) forwarding in
more pieces, if the forwarder accepts IPv6-only and
forwards it to IPv4-only, the receiver can't send DSNs
or ordinary replies. Or if they can over another route
that is something an 1123 5.3.6(a) forwader can't know.
Consider it as one-way spam if the mail with IPv6-only
addresses somehow makes it into IPv4-only land. That
is broken, as you said, but unrelated to demanding an
MX record for IPv6 SMTPs.
Without a mandatory MX for your IPv6 SMTP if the mail
reaches IPvAnything land and folks want to reply or
send DSNs where required, they have to query for MX,
A, and AAAA to finally find your IPv6 SMTP.
For simple "if it can't receive it has no business to
send" checks at the border it is also three queries.
With a mandatory MX for IPv6 we simply reduce this.
All "v=spf1 -all" and obscure null-MX ideas could be
phased out if "no MX" means "cannot receive, must not
send". We will never reach this ideal for IPv4, but
*NOW* is a chance to prepare it for the time when the
whole Internet is IPv6-only.
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