Re: current usage of AAAA implicit MX?

At 22:03 +0200 on 04/05/2008, Arnt Gulbrandsen wrote about Re: 
current usage of AAAA implicit MX?:
> Peter J. Holzer writes:
> > On 2008-04-04 17:30:29 -0400, Robert A. Rosenberg wrote:
> > > RFC 3974
> > > [...]
> > > It, BTW, REQUIRES an MX if there are any IPv6 MTAs  and only 
> > > supports A-Fallback in the absence of an MX
> > No.
> "For the transition period, all mail domains should have MX records 
> such that MX targets with IPv4 and IPv6 addresses exist"?
I will point out the existence of an MX for the FQDN makes the 
question of AAAA-Fallback (or the existence of AAAA records for FQDN 
when you simulate the existence of a FQDN MX 0 FQDN record) a moot 
point since there is no AAAA record to fall-back on since it is 
LISTED in the actual MX as a valid MTA. Also the quote above says 
"SUPPLY MX RECORDS for your MTAs".
> > >  It also states that in the presence of an MX containing IPv6 address
> > >  references, an IPv4-Only stack should ignore the IPv6 addresses
> > Yes, of course. It cannot reach them anyway. Just as IPv6-only 
> > hosts can ignore A records since they cannot reach them either.
> An IPv6-only host must use the presence/absence of A records to 
> generate the DSN. The DSN status code is 5.1.2 if there's no A and 
> 5.4.x if there is an A but no way to reach it (ie. no configured 
> relay).
> Arnt
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