John C Klensin wrote:
if we stick with Option 1 and the language isn't clear enough,
the relevant sentence can be supplemented with an explicit
statement to the effect that any values that point to CNAMEs
that appear are nonconformant and out of the scope of the
standard.
That would be good and hopefully address Trevor's concerns in
<http://thread.gmane.org/gmane.ietf.smtp/6517/focus=6525>
Paul Smith proposed 2007-12-14 (getting two ACKs):
| 2821bis should state that "putting CNAMES as the target of
| MX records is not allowed, as stated in RFC 2181"
the historical reason for the prohibition on names that point
to CNAME RRs in the data field of MX RR was, I believe, to
reduce the risk of looping within the DNS.
The explanation in RFC 974 is more convoluted, something about
figuring out what's LOCAL and what's REMOTE.
Frank