Re: Minor is. It's not a pardigm change

> - mail queues get clogged up trying to send mail to hosts that don't 
> support SMTP.  because "connection refused" is a temporary error to 
> SMTP, any mail sent to a host that doesn't support SMTP will sit in a 
> queue for days before bouncing or being discarded. 
This one is the one which I find the most compelling. In my mind, at the 
moment the specification causes ambiguous behaviour because of the 
implicit fallback to A records. Does 'connection refused' mean 'there is 
no mail server here, so give up and don't come back' or 'the mail server 
is down for maintenance/crashed/etc, so please do come back'? With the 
implicit fallback to A, there's no way to tell, but with a required MX 
record there is.
Spam being received by people won't be reduced by not allowing fallback 
to AAAA, as the mail wouldn't get anywhere anyway, but it will be 
sitting in queues on servers taking up resources unnecessarily.
I can see the need for this ambiguity in IPv4 now for backwards 
compatibility reasons, and I can see why it was there originally for 
backwards compatibility at that time, but now there is the prospect of 
clearing things up and removing the ambiguity in the future, without 
breaking backwards compatibility more than it would be anyway (IPv4 only 
mail servers won't be able to talk to IPv6 only servers anyway).
*Now* is the time to do this. If the IPv6 mail standard is finalised 
allowing fallback to AAAA, then the cat is out of the bag, and there's 
no going back. So, unless there's a good technical reason (the only 
reasons I've seen so far have essentially been 'ease of use' or 
'political' reasons (the possibility of overly time consuming approval 
processes to get another MX record added in the rare case where an 
existing host without mail will need to have mail added)) I'm for not 
allowing fallback to AAAA as it removes the 'connection refused' ambiguity.