I think we all agree that was a mistake, and CNAMEs are fine
in MAIL and RCPT and you shouldn't mess with them, but I can't
find any place this advice has been explicitly deprecated.
Try Section 2.3.5 of RFC 5321 (or its predecessor in 2821, which
was a tad less explicit):
this particular case. YMMD, in which case specific suggestions
for what needs to be fixed and how to fix it would be welcome.
Putting on my nitpicky lawyer hat, I note that RFC 1123 is a full
standard. RFC 2821 is a proposed standard obsoleted by RFC 5321 which is
a draft standard which updates but does not obsolete 1123.
So my reading is that while 5321 tells me that it's OK to use a CNAME in
MAIL FROM/RCPT TO, 1123 still tells me that it's better to resolve those
CNAMEs first.
I hope that nobody who matters believes this, but if you look at the two
documents it's not a totally ridiculous way to reconcile them. Should we
open up 5321 again, I think a sentence specifically revoking the CNAME
advice in 1123 wouldn't hurt.
R's,
John
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