Registration model, 2821bis-06

Now that I think about it, 2821bis-06 does require something more than 
A/AAAA in order to operate a mail server: If a host only has an IPv6 
address, 2821bis-06 section 5.1 requires the operator of that host to 
form an arrangement with a dual-stack relay in order to send mail to 
IPv4-only targets.
IMO, that's not an unreasonable requirement. I think it's perfectly 
reasonable and almost unavoidable. I'm pointing it out to show that the 
text already requires something more than getting an AAAA record and 
setting up some software on the host itself.
How many people would be happy if 5.1 were to say this?

 - if there is no MX record, but there is an A, then the sending
   host MUST synthesise an MX
 - if there is no MX record and no A, but there is an AAAA, then
   the sending host MAY synthesise an MX
 - even if a client doesn't use AAAA records for synthesising MX
   records, it MAY use them for delivering mail once an MX has
   been synthesised.

This permits people to set up IPv6-only mail servers without any MX or 
relay, and it doesn't declare any deployed 2821-compliant code to be 
illegal, and it doesn't tell anyone to assume that IPv6-only devices 
are mail servers by default.
Connectivity to/from AAAA-only mail servers will suck terribly for many 
decades to come, but no 2821bis wordsmithing can avoid that. Only an 
arrangement with a dual-stack relay can improve connectivity, and that 
practically requires an MX RR or more.
Arnt