Benny Pedersen wrote:

[About CNAME MX records...]

>> rfc means 'request for comment'. and rfc's change as technology changes.

>
> but not much in smtp have changed since first version deployed


The RFC in question (RFC2181) is about DNS, not SMTP.

Actually, in STD0010 and STD0013 (the standards documents
describing SMTP and DNS), there is no clear prohibition against
CNAME MX records.

There is this in STD0010:
---8<---
There is one other special case. If the response contains an
answer which is a CNAME RR, it indicates that REMOTE is actually
an alias for some other domain name. The query should be repeated
with the canonical domain name.
---8<---

Using a CNAME MX record might break the standards track proposal
RFC2181, but (AFAICS) it does not break the actual standards
(STD0010, STD0013). OTH, not resolving a CNAME MX record to the
actual A record does break the SMTP standard (STD0010) from what
I can see.

Note:
I only browsed STD0010 and STD0013 now, and one of the
improvements in later RFCs is the use of MUST and SHOULD to make
requirements and suggestions easier to distinguish. So if this
matters to you, read the documents.

References:
STD0010:
STD0013:
RFC2181:

Appendix:
This really has little bearing on wether one can refuse to accept
a mail with a sender domain the MX of wich points to a CNAME. Of
course one can refuse to accept such a mail. One can refuse to
accept mail based on the air humidity in Glasgow and the
temperature at Luleå if one wants to.

Regards
/Jonas
--
Jonas Eckerman, FSDB & Fruktträdet
http://whatever.frukt.org/
http://www.fsdb.org/
http://www.frukt.org/