On Oct 6, 2019, at 10:07 PM, John C Klensin <john-ietf(_at_)jck(_dot_)com>
Ah, ok. Makes sense. Probably even better to try to treat this
as a service, e.g., make the leftmost label "_smtps" or even
"_dombox" or the like. I haven't thought deeply about it, but
that model, which is reasonably well-defined, should work as
long as _dombox.mail.example.com. is treated as the name of a
host because, in the DNS record were, e.g.,
mysystem.example.com. MX 0 _dombox.mail.example.com.
it has to be an alias for something else or if it has to be
parsed in a special way, that would probably violate the SMTP
rule about MX targets.
That's an invalid hostname, and I would not be surprised to
find a non-trivial number of MTAs that are not only unable to
deliver email to a domain with MX hosts of that form, but may
even reject inbound mail when a sending domain has no valid MX
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