Paul Smith wrote:
IMHO, 2821bis should state that "putting CNAMES as the target of MX
records is not allowed, as stated in RFC 2181"
Paul, I generally agree with your other points in your message.
But this still won't change the BCP requirement for "Industry Ready"
software to recognize it. So even if it was stated strongly, what you
will end up doing is someone new who will implement it as stated and
then get all bent out of shape when he comes across those CNAME
situations in the real world.
Guess who has to change?
While he might get on mission and initially succeed in getting DNS
administrators to change their setups, eventually he will get tired of
it and begin to recognized and add support for it - bitching and moaning
at the same time. If he doesn't - he simply isn't normal. <grin> The
fact is, sometimes it is not feasible for DNS setup to be changed.
So IMO, -1 on changing RFC 29821bis.
Consider the fact it is already general practice for DNS software to
recognize CNAME issues, including but not exclusive to:
- Watching and Limiting CNAME loops 
- Resolving CNAMES from A records lookups  
Yes, it is expected MX resolves to A records , the main concern being
that it will cause extra queries/network burdens and early resolvers did
not expect it.
But due to the nature of the world, those A records can easily end up
being CNAME RRs . It is prudent that Industry Ready software be
prepare for that. This SMTP Operational Experience is officially
recognized in RFC 3974 
Mandating it in RFC 2821bis isn't going to change that. Even if you are
looking at future generation DNS admins doing things correctly for MX
resolution, I still say the DNS client software will still need to take
into account the MX resolution will point to CNAMES, even if the
practice is deprecated.
Hector Santos, CTO
 RFC 2181.
 See RFC 974 for historical perspective.
 RFC 3974 SMTP Operational Experience in Mixed IPv4/v6 Environments