At 16:11 +0200 on 03/31/2008, Frank Ellermann wrote about Re: Minor is. It's not a pardigm change:
No matter what 2821bis will say, when IPv6 enters the picture they need to talk. The mail guy in an IPvX network will want mail from non-IPvX networks. Frank
So if you are a IPv6 SMTP MTA document this fact with a MX. If you are IPv6 but NOT an SMTP MTA document this fact either with an "MX ." or better with the absence of an MX for that FQDN.
IOW: If a FQDN has no MX, there should NOT be an attempt to find a MTA Server for that FQDN by doing an AAAA fallback (but doing a fall-back to an A is acceptable for historic reasons). Failure to find an MX or an A for a FQDN should act as it does at present (IOW: In the absence of an MX do NOT assume that an AAAA might represent a MTA). Saying NO AAAA Fall-Back is the only way to avoid the problems that A Fall-Back causes since there is no VALID need for AAAA fall-back in the absence of an MX unlike the situation prior to the creation of the MX record type that necessitated doing A Fall-Back (since prior to the MX, the A WAS the way to find a MTA).
