On Mon, 2023-02-27 at 18:53 -0700, Luke via mailop wrote: > 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: > > 452 Sender Rejected:
Keeping in mind that the RFC says not to use the text in your decision, these two sound the most reasonable. Skipping for brevity. > 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. This could postfix's unverified_recipient_reject_code. When (for example) postfix sits in front of an Exchange server and when a full list of valid recipients is not available, postfix can launch a background probe to see if the recipient is valid in Exchange before it will accept a message destined for it (otherwise, you get backscatter). If postfix is up but Exchange is down, it'll give you this rejection. > 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. This happens when somebody's domain expires and the MX record gets pointed somewhere it shouldn't. This is the right response because when they finally find the guy who knows the guy who knows the guy who receives the email that resets the password at the registrar three days later, it's nice to have all their mail start flowing as if nothing ever happened. > 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 Not pounding on servers that have told you to go away with a 5xx has obvious benefits for both sides though. What's the downside to leaving the 4xx in the queue for a while? No one is going to fault you for retrying a message they told you to retry, and even 1% of legitimate mail is a lot to toss out without a good reason. Finally, while it might be 1% of *your* mail it's not 1% of *our* mail. Michael Scott said Wayne Gretzky said that you lose 100% of the messages you don't retry to recipients who would have accepted them. _______________________________________________ mailop mailing list mailop@mailop.org https://list.mailop.org/listinfo/mailop
