Chaining MX records illegal?

[Gregory Hicks]

> From: Barry Margolin <barmar at alum.mit.edu>
> Subject: Re: Chaining MX records illegal?
> Date: Thu, 10 Nov 2005 01:12:20 -0500
> To: comp-protocols-dns-bind at isc.org
> 
> In article <dku9b6$20ih$1 at sf1.isc.org>,
>  Chris De Young <chd at arizona.edu> 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?
> 
> There's nothing invalid about it, but it won't do what I think you 
> expect it to do.  Mail for chud.net will be delivered to 
> mail.foobar.com, not smtp.yahoo.com.  After looking up an MX record, 
the 
> sending system will then look up the A record of the name it gets, it 
> shouldn't look for an MX record of it.

To expand a bit...

As Barry said, that is not really "chaining" of MX records.  These are 
two SEPARATE MX records.  They are not related to each other even though 
the first points to the second.

This works as:

If you are sending mail to 'chud.net', my mail server is 
"mail.foobar.com".  (look up A record)

If you are sending mail to a user at mail.foobar.com, my mail server is 
smtp.yahoo.com...  (Look up A record).

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