At 23:51 +0200 on 04/07/2008, Frank Ellermann wrote about Re: IPV6:
Where are we? (SMTP):
Hector argues that it's the job of IPv6 domains to be reachable,
for that they must either also offer A, or use an explicit MX
with an IPv4 SMTP at a third party in addition to their AAAA.
Assuming that 2821bis will be as long a standard as STD 10
(about 30 years) adding this pragmatical idea could backfire
when IPv4 is remote history.
You are assuming that IPv6 domains will not initially be Dual Stack.
So long as they are dual stack (or have someone who is dual stack act
as a secondary MX to forward to the IPv6-Only MTA) it will work. You
FQDN MX 10 IPv4
MX 20 IPv6
with both the IPv4 and IPv6 MTAs being able to save to the IMAP/POP
mailboxes or both set to hand off to an internal MTA for final
As IPv6 becomes more widespread make the IPv4 MX a 30 not a 10 and
the IPv6 will become the preferred MTA.
Eventually you can remove the IPv4 (if there is a flag-day for
depreciation of IPv4) or keep running the IPv4 or farm it out to a
dual stack who will hand off to the IPv6 (as above).
Once you use a IPv4/IPv6 MX, the only use of the A will be by a
antiquated resolver/MTA that does not understand MX or has its MX
support turned off for some reason. All others will locate and talk
to the IPv4 MTA via the MX.