For better or worse, *not *retrying *some *4xx is even easier to justify.
Here are a few massive examples.

421 4.7.1 Message permanently deferred: less than 1% delivered rate after
32 retries. This is across 100s of millions of messages, thousands of
unique senders, and thousands of IPs, looking back over 6 months of data.
Retrying is a waste of resources on both sides.

452 Sender Rejected: we get this from thousands of distinct receivers. Same
results as the above. Less than 1% delivery rate after 32 retries over a
diverse sampling of senders.

450 4.1.1 Recipient Address Rejected: this one is tough because with some
MXs retries result in delivery, in others, it's a dead end. Dynamic rule
handling per receiving MX would be awesome, but it would require machine
learning to accomplish at scale.

451 Relay Not Permitted: this one can mean a lot of things and comes in a
few flavors, but when the data shows retrying never results in a delivery,
we permfail it.

Unlike the rare 5xx we decide to retry, these 4xx that shouldn't be retried
are far more numerous and come from large-ish reputable receivers. Believe
me, simply retrying 4xx and failing 5xx would make our jobs a lot easier.
Unfortunately the real world isn't so simple

Luke

On Mon, Feb 27, 2023 at 5:15 PM Michael Orlitzky via mailop <
mailop@mailop.org> wrote:

> On Mon, 2023-02-27 at 15:45 -0700, Luke via mailop wrote:
> >
> > With that said, given the responses in this thread, we will be taking
> > a close look at the few rules we have in place where we retry 5xx and
> > see if A.) the rules are still being hit at all, and B.) are these
> > retries still resulting in reliable and consistent delivery after a
> > small number of attempts.
> >
>
> That's a good idea anyway (thank you), but the original issue wasn't
> about an extra retry or two for a 5xx, but the lack of any retry at all
> for a 4xx.
>
> That happens all day every day and is what makes mail go missing.
>
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