Quoting John Levine via Exim-users (exim-users@???):
> >That text is obsolete. There is no such text in RFC5321, and
> >CNAME-valued mail domains have long been OK. The sending MTA (its DNS
> >resolver) is expected to restart the MX lookup at the target of the
> >CNAME, and if no MX records are found, use the final A/AAAA records.
>
> Huh, you're right. In my minor defense, the CNAME code was written in
> 1998, RFC 2821 was published in 2001, and as far as I can tell this is
> the first time in 20 years that I've sent to a CNAME'd domain that
> caused a problem. I'll go patch the code.
Exim has the same CNAME "bug" left over from those times.
https://bugs.exim.org/show_bug.cgi?id=1383
Though it works in different mysterious ways routing bounces back as can
be seen in the examples in that bug report. ;)
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