[dns-operations] Why would an MTA issue an ANY query instead of an MX query?

Colm MacCárthaigh colm at stdlib.net
Sun Jun 10 16:50:55 UTC 2012

SMTP predates the "MX" rrtype. In RFC821, "colm at stdlib.net" means
"connect to the host stdlib.net" and host records (e.g. "A" and now
"AAAA") are what matters [*]. RFC1123 and later RFC2821 regularised
the "MX" rrtype for mail routing, obviating the need for the host
records at a mail domain.

However, the correct SMTP behavior, at least considered by many, still
is that if you don't find an MX record for a domain name, try to find
a host record. From the point of view of an SMTP server, an "ANY"
query is a  rational way to find all of the records it will need, in
one pass.

[*] Well at various times, MD, MF, MAILA and MAILB were all relevant
too, but those have fallen completely out of disuse.

On Sun, Jun 10, 2012 at 3:17 AM, Dobbins, Roland <rdobbins at arbor.net> wrote:
> Clue appreciated, thanks!
> -----------------------------------------------------------------------
> Roland Dobbins <rdobbins at arbor.net> // <http://www.arbornetworks.com>
>          Luck is the residue of opportunity and design.
>                       -- John Milton
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