It would definitely create a problem if you were to point your
MX records at
CNAME records since it is against the standards. The clearest explanation is provided by RFC2181 §10.3:
10.3. MX and NS records
The domain name used as the value of a NS resource record, or part of
the value of a MX resource record must not be an alias. Not only is
the specification clear on this point, but using an alias in either
of these positions neither works as well as might be hoped, nor well
fulfills the ambition that may have led to this approach. This
domain name must have as its value one or more address records.
Currently those will be A records, however in the future other record
types giving addressing information may be acceptable. It can also
have other RRs, but never a CNAME RR.
Searching for either NS or MX records causes "additional section
processing" in which address records associated with the value of the
record sought are appended to the answer. This helps avoid needless
extra queries that are easily anticipated when the first was made.
Additional section processing does not include CNAME records, let
alone the address records that may be associated with the canonical
name derived from the alias. Thus, if an alias is used as the value
of an NS or MX record, no address will be returned with the NS or MX
value. This can cause extra queries, and extra network burden, on
every query. It is trivial for the DNS administrator to avoid this
by resolving the alias and placing the canonical name directly in the
affected record just once when it is updated or installed. In some
particular hard cases the lack of the additional section address
records in the results of a NS lookup can cause the request to fail.
You might be able to find anecdotal evidence through search engines that some DNS and MTA software support this, but that should be considered the exception and not the rule. Lack of this support will not be considered a bug by most software authors. Always avoid pointing a
MX record at a
The biggest problem you face right now is that the TTL for the
MX records in your example are all three weeks, and your change is next week. I strongly recommend that you request that this switchover be delayed, and lower your TTLs to somewhere in the neighborhood of ten minutes. You can raise the TTL again once the cutover is complete.