Oops, second paragraph should read: The header does *not* ...
> On Mar 30, 2019, at 3:05 PM, Viktor Dukhovni <ietf-d...@dukhovni.org> wrote:
>
> 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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