On Mon, Aug 25, 2025 at 02:57:28AM +0200, Ángel via mailop wrote:
> On 2025-08-24 at 19:39 +0100, Simplelists - Andy Beverley wrote:
> > I've just checked now and interestingly the deliveries to the same
> > Gmail
> > server immediately before and immediately after succeeded. If it had
> > been an actual DNS error I would have expected more than one failure
> > because of caching (although who knows what's the other side of that
> > single IP address)
>
> Not necessarily. I expect there is a dns caching server which is
> returning a failure due to timeout, when it goes up the gmail stack, it
> somehow gets converted into an empty list, not remembering that there
> was an error (or perhaps they do that willfully, because they don't
> want to 400 missing rptr which don't return anything?)
> But on the next email, the delayed got into the cache, so it's now
> served immediately.
The first thing to keep in mind is that Google have load-balanced DNS
servers and many load-balanced inbound MTAs. So the DNS answers for the
three deliveries could have come back from different DNS resolvers.
But, assuming that there was actually a transient (SERVFAIL, timeout,
... or equivalent) issue at the resolver, it would be a serious bug for
the SMTP layer to equate SERVFAIL with NODATA.
> It may even happen that the timeout happens at the higher-level code
> that queries the DNS stack, not inside the DNS resolver.
Again, failure != NODATA, and failing to make the distinction is a
serious bug.
> > > Had the relevant DNS queries resulted in timeouts or other
> > > transient lookup problems, and the remote MTA were Postfix, the
> > > reject code would have been a 4XX. I don't know that Gmail gets
> > > this right, but I'd be surprised if they got it wrong, that'd be a
> > > serious bug, worth contacting their engineering team about.
> >
> > Anyone have a contact...?
>
> Brandon is in the mailing list, he sometimes filled the needed
> internal google tickets.
If Brandon is on the list, and still reading this thread, perhaps
he'd be able/willing to shed some light on this apparent puzzle.
--
Viktor. 🇺🇦 Слава Україні!
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