Re: Last Call: draft-klensin-rfc2821bis

Frank Ellermann wrote:
> Bill Manning wrote:
>  
>> example.com.  soa (
>> stuff
>> )
>  
>> ns foo.
>> ns bar.
>> ;
>> 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@example.com
> at their border for obvious reasons.

uh, no.  IPv4-only hosts can see the AAAA record even if they can't 
directly send mail to that address.  and there's no reason ("obvious" or 
otherwise) why a MTA should reject mail from a host just because that 
MTA can't directly route to it - since there's nothing that requires a 
user to use the same MTA for outbound mail that is used to receive 
inbound mail.  (granted, lots of really really stupid things are done in 
the name of spam filtering that make no more sense than this, but 
they're not "obvious" - they're just stupid).

of course, as a practical matter, any domain for which mail eventually 
ends up in an IPv6-only world is going to need to have at least one MX 
pointing to a IPv4-capable host, until IPv6 becomes ubiquitous or very 
nearly so.  so the example above only makes sense in a world where IPv6 
is ubiquitous - or perhaps, where the owner of example.com wants to 
gauge the degree of IPv6 support for outgoing mail.

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