From: Wietse Venema (wietseporcupine.org)
Date: Wed Mar 26 2008 - 10:14:30 CDT

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Momo:
> Hi folks,
>
> I'm trying to find info on IP address selection in a DNS RoundRobin setup.
>
> In the first case 2 MTAs , 2 same weight MX records (1 IP address each):
> I know that if the first host fails, the second MX will be tried immediatly.
>
> Now in the second case, 2 MTAs, only 1 MX record, but 2 IP adresses
> resolving on it.
> If the first host fails, will the second one be tried immediately, or
> will the message be queued on the sending MTA, and retried later with
> the hope the MX record will be resolved on the responding host on next
> try?

This depends on the MTA implementation.

RFC 2821 assumes that one MX record corresponds with one host.
Consequently, some MTAs try only one IP address per MX record (*).
Postfix, however, tries each A record under an MX record (**).

So it may be a good idea to use multiple MX records.

> Is there any RFC describing retrying behaviour?

SMTP is described in RFC 2821.

        Wietse

(*) This MTA behavior seems to contradict RFC2821, which says:

   The destination host (perhaps taken from the preferred MX record) may
   be multihomed, in which case the domain name resolver will return a
   list of alternative IP addresses. It is the responsibility of the
   domain name resolver interface to have ordered this list by
   decreasing preference if necessary, and SMTP MUST try them in the
   order presented.

(**) But it randomizes all IP addresses with equal MX preference.

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