> On Sep 18, 2016, at 6:47 PM, Jim Fenton <fen...@bluepopcorn.net> wrote:
> Yes; I'm not sure why I singled out MX and CNAME because I know those
> aren't the only ways of locating the server. I would propose to change
> "confirm that any MX record or CNAME lookup used to locate the SMTP
> server" to "confirm that and DNS record lookup(s) used to locate the
> SMTP server".
No other DNS records are used by MTAs to locate the nexthop relay
for a destination domain. SRV records are used exclusively (and rarely,
given the paucity of adoption of RFC6186) by MUAs. Since it is the
MUA that signals the required TLS policy in the first place, the MUA
also determines what constitutes sufficient security for the initial
MUA -> MSA hop. It is not clear that this specification, which I think
defines the responsibilities of the MTA on the receiving end of this
policy, need also cover the responsibilities of the MSA.
Sorry, have not yet read the draft, more comments at that time.
I still think that any such mechanism needs to be able to not only
request a greater protection for a given messages, but also to request
lesser protection, prioritizing delivery over security. On the
MUA -> MSA link, the administrator may, as a matter of local policy,
configure a policy to refuse messages that request security downgrades.
Other MTAs would not have such liberty.
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