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

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