MX choosing

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

Phil Mayers <p.mayers at> wrote:
> On 07/22/2011 09:50 AM, Feng He wrote:
> >
> > Given the MX hosts for domain:
> >
> > $ dig mx +short
> > 5
> >
> > $ dig +short
> > [ 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
but others (who believe that multihomed servers exist) disagree. RFC 5321

  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.

f.anthony.n.finch  <dot 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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