It appears that Viktor Dukhovni <ietf-smtp(_at_)ietf(_dot_)org> said:
Indeed their implementation is flawed. They should either streadfastly
refuse to deliver email to all domains where the the MX is a CNAME, and
so outside the interoperability scope of RFC5321. Or, if like most
MTAs, they're pragmatic and allow CNAMEs, the CNAME should not affect
the logical MX host name used with MTA-STS, just as would be the case
with web browsers, etc.
It seems to me pragmatic to say "we think we understand what you think you're
but it's still wrong."
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