[dns-operations] MX record definition?

[SM]

At 15:21 17-12-2008, Seth Mattinen wrote:
>Does anyone know offhand where the MX record is defined? The best I 
>can find is section 3.3.9 in RFC 1035. I'm trying to find a 
>reference that says numeric MX records are bad. I'm assuming they're 
>a no-no, at least that's what I've always been told.

Section 5.1 of RFC 5321 explains how the SMTP client does address 
resolution and mail handling.  Quoting that RFC:

    When a domain name associated with an MX RR is looked up and the
    associated data field obtained, the data field of that response MUST
    contain a domain name.  That domain name, when queried, MUST return
    at least one address record (e.g., A or AAAA RR) that gives the IP
    address of the SMTP server to which the message should be directed.

Regards,
-sm