This (reparagraphing) change has been tentatively made to -08.
I can also change the final sentence to read "The DNS DATA field
associated with the lookup of an MX record must not contain a
domain that, in turn, is associated with a CNAME record" or
something to that general effect if people are convinced it
would be more clear rather than more confusing.
--On Thursday, 21 February, 2008 11:16 -0500 Tony Hansen
Alessandro Vesely wrote:
> Derek J. Balling wrote:
>> I know this may be a dead horse, but...
>> "The result of an MX lookup MUST NOT be a CNAME."
>> Can this *please* be slightly reworded? "The RR value of
>> lookup..." perhaps?
It is the compound "MX lookup" that generates ambiguity, as
it is used to indicate the initial lookup of the domain name.
Actually, that's what the whole first paragraph in 5.1 is
talking about. If it were considered a minor change, I'd
propose moving that sentence to the end of the next
paragraph, where its rationale can be grasped more easily.
<techie hat on>
I'm thinking that the problem really lies in the paragraph
being so long and covering multiple steps. If it were split
apart like this, I think it would be more obvious where each
statement applies in the flow:
Once an SMTP client lexically identifies a domain to which
be delivered for processing (as described in sections
and Section 3.6), a DNS lookup MUST be performed to
domain name (RFC1035 ). The names are expected to be
qualified domain names (FQDNs): mechanisms for inferring
partial names or local aliases are outside of this
Due to a history of problems, SMTP servers used for initial
submission of messages SHOULD NOT make such inferences
Submission Servers  have somewhat more flexibility) and
intermediate (relay) SMTP servers MUST NOT make them.
first attempts to locate an MX record associated with the
name. If a
CNAME record is found instead, the resulting name is
processed as if
it were the initial name. If no MX records are found, but
RR (i.e., either an IPv4 A RR or an IPv6 AAAA RR, or their
successors) is found, the address RR is treated as if it
associated with an implicit MX RR, with a preference of 0,
to that host.
If one or more MX RRs are found for a given name, SMTP
systems MUST NOT utilize any address RRs associated with
unless they are located using the MX RRs; the "implicit
above applies only if there are no MX records present. If
are present, but none of them are usable, this situation
reported as an error. The result of an MX lookup MUST NOT
</techie hat off>