I do have an opinion and I think I can back it up with quotes from the
RFC, but I've tried to state the question neutrally.
Back in 1987 when the DNS was young, CNAMEs were invented to describe
short nicknames for long host names, or as a transition aid when a
host's name was changing. The rule for SMTP was that if you saw a
CNAME you should rewrite it to the CNAME's target, since the CNAME was
just a nickname for the host's real name.
These days, perhaps 1% of CNAMEs are used that way, and the other 99%
are used to do long term DNS management, by allowing a name in one
part of the DNS tree to alias to a name somewhere else to outsource
the management of the first name to the second. In this usage,
rewriting CNAMEs is obviously not a good idea.
While you will still find people who insist that you should rewrite a
CNAME in a mail transaction (do not ever, EVER, ask this questionon
the dnsext list) they are wrong.
RFC 5321 still says that the HELO/EHLO name has to resolve directly to
an A or AAAA, not a CNAME. I don't know if anyone pays any attention
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