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. > > _______________________________________________ > mailop mailing list > mailop@mailop.org > https://list.mailop.org/listinfo/mailop >
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