If we don't specify some RFC/BCP to specify how SMTP over IPv6 should be
negotiated, then no one will follow.
The IETF is amazingly resistant to making v6 SMTP different from v4 SMTP
in any way.
In particular, I suggested that they not have a rule for fallback to
AAAA in the absence of MX. The rationale is straightforward: most
hosts with AAAA (and indeed A) records are not mail servers, people
need to add new DNS records anyway for v6 so the incremental effort to
install MX records is quite small, and it'll make mail more reliable
by making it easier to tell when a domain doesn't receive mail. They
all said too late, there are already a handful of v6 mail hosts.
Sigh.
R's,
John
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