Chaining MX records illegal?

[Kevin Darcy]

Chris De Young wrote:

>Hi,
>Am I correct in my interpretation that chaining together MX records is illegal,
>despite the fact that it seems to mostly work?
>
>For example:
>
>chud.net	mx 10	mail.foobar.com.
>.
>.
>.
>mail.foobar.com	mx 10	smtp.yahoo.com.
>
>is a no-no?
>
As an Internet Standards issue, this is a fairly interesting question. 
After defining the structure of the MX Resource Record, RFC 1035 punts 
to RFC 974 as far as how MX records are *used* (i.e. semantics rather 
than syntax), and the successor to RFC 974, RFC 2821, simply says "When 
the lookup succeeds, the mapping can result in a list of alternative 
delivery addresses" without specifying whether those "addresses" must be 
exclusively A records or can be MX records or, presumably even something 
else like SRV records. One must also take with a grain of salt the 
singular number of "the lookup" in the quoted text above, since even in 
a "normal" case there's no guarantee that all of the A records 
associated with the MX records in the Answer Section of the MX-query 
response will be present in the Additional Section of the response, so 
the SMTP client might have to make one or more subsequent queries to 
translate those MX targets into A records anyway.

As a practical issue, though, why would you want to tempt fate by making 
your mail delivery dependent on a construct that was apparently not 
intended or foreseen by the standards writers and/or implementors, and 
at best only "mostly" works? I don't see any benefit to it.

                                                                         
                                                      - Kevin