MX Record Theories

Tue May 26 19:01:11 UTC 2009

On Tue, 26 May 2009 11:03:59 PDT, gb10hkzo-nanog at yahoo.co.uk said:
> would be most interested to hear NANOG theories on the variety of MX
> record practices out there, namely, how come there seem to be so many
> ways employed to achieve the same goal ?

The trick here is that it isn't always *exactly* "the same goal".  There's
multiple mail system architectures and design philosophies.

One often overlooked but very important design point for the *large* providers:

% dig aol.com mx
;; ANSWER SECTION:
aol.com.                2805    IN      MX      15 mailin-01.mx.aol.com.
aol.com.                2805    IN      MX      15 mailin-02.mx.aol.com.
...
;; WHEN: Tue May 26 14:40:41 2009
;; MSG SIZE  rcvd: 507

That 507 is critically important if you want to receive e-mail from sites
with fascist firewalls that block EDNS0 and/or TCP/53.  5 bytes left. ;)

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