Re: Registration model, 2821bis-06

Arnt Gulbrandsen wrote:

> 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.
first rule is fine - really I think that fallback to A should eventually 
be deprecated (probably as of some flag day) but 2821bis clearly is not 
the place to do this.
second rule - MAY should be SHOULD NOT, perhaps even MUST NOT.

third rule is a bad idea, IMHO.  the 974 fallback means that no MX 
records + an A record implies authorization to deliver mail to that IPv4 
address.  it doesn't imply any authorization to deliver mail to any IPv6 
address.
the principle of least surprise should dictate that AAAA use is 
consistent from one conforming sending MTA implementation to another. 
the presence or absence of an A record should not affect whether an AAAA 
record is used.
I envision an MTA administrator getting IPv6 support for his server and 
then testing to see if it works for receiving mail.  IPv6 should work 
for receiving mail if the DNS is configured correctly - i.e. an MX 
record is present - and not otherwise.