> On Mar 30, 2019, at 2:44 PM, Benjamin Kaduk <ka...@mit.edu> wrote:
>
> This doesn't really say anything about or give guidance to intermediate
> MTAs. (Do we want to differentiate between initial and intermediate MTAs,
> too?)
Essentially, all MTAs are intermediate MTAs. The header is added to the
message via the sender's *MUA*, and conveys the same sender preference to
every SMTP relay (MTA) en-route. For this header to be effective at
achieving its goal, there needs to be no prior difference between the MSA's
first relay hop and further downstream relays. TLS-related delivery issues
can occur at any hop along the path from sender to recipient.
The header does obligate every en-route MTA to ignore its local policy.
Where such local policy mandates TLS, it will likely trump the header.
Where the MTA has no explicit policy and would otherwise follow the
hints (DANE or MTA-STS) from the next-hop destination, the header should
generally allow delivery when operational errors would otherwise break
such delivery by promising and not delivering on the requirements of DANE
or MTA-STS.
--
Viktor.
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