MX choosing

Tony Finch dot at dotat.at
Fri Jul 22 09:58:50 UTC 2011


Phil Mayers <p.mayers at imperial.ac.uk> wrote:
> On 07/22/2011 09:50 AM, Feng He wrote:
> >
> > Given the MX hosts for sympatico.ca domain:
> >
> > $ dig sympatico.ca mx +short
> > 5 mxmta.sympatico.ca.
> >
> > $ dig mxmta.sympatico.ca +short
> > 67.69.240.17 [ and several others ]
> >
> > when the peer MTA fail to talk to one of these hosts,will it try the
> > next one? or it just give up?
>
> It should try the next one, but there is a lot of crappy MTA software out
> there. See section 5 of RFC 5321 [...]

I agree that it is better to immediately retry using the other IP
addresses (and in fact our setup assumes they will - see mx.cam.ac.uk)
but others (who believe that multihomed servers exist) disagree. RFC 5321
says:

  The question of whether a sender should attempt retries using the
  different addresses of a multihomed host has been controversial. The
  main argument for using the multiple addresses is that it maximizes the
  probability of timely delivery, and indeed sometimes the probability of
  any delivery; the counter-argument is that it may result in unnecessary
  resource use.

Tony.
-- 
f.anthony.n.finch  <dot at dotat.at>  http://dotat.at/
East Dogger, Fisher, German Bight: Northwesterly 6 to gale 8. Moderate or
rough, becoming rough or very rough. Rain. Good, becoming moderate or poor.



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