If people intend to be interoperable, there is NO variation in what a 4xx and a 5xx mean.
4xx means: this message can be queued and retried at a future date. 5xx means: this message cannot be retried without human intervention. Those interactions are defined in RFC 821 and it’s successors. [1] The variation / confusion / problems / conflict comes when we try and use these very clear *transactional* signals to interpret what to do with a different and future message to the same address. Bounce handling in the transaction should follow the RFC. If the SMTP response starts with a 4, queue it and retry. If the SMTP response starts with a 5, stop trying to deliver it. Address handling and list hygiene is undefined in the SMTP spec. It’s something we’ve tried to cobble together with every group doing a different thing over the years. It’s a mess and it is complicated and there probably should be documentation of what to do with it. [2] laura [1] I’m still annoyed by “4xy: this message will never deliver” that a major ISP uses. If the message will never deliver, give it a 5xx. [2] Yes, this is my current project - figuring out a way to make bounce handling less of a maze of twisty little codes all alike and making it a clearer communication channel. > On 26 Feb 2023, at 05:12, Evan Burke via mailop <mailop@mailop.org> wrote: > > > There's a lot of variation in what 4xx and 5xx mean in practice. This means > there's room for some senders to spend time investigating and understanding > what each receiving domain's 4xx and 5xx (and subcategories thereof) actually > mean for address delivery over time, and for designing rules and policies to > handle them based on observed outcomes - IF those are a more reliable > indicator than the general guidance from RFCs. > > That doesn't mean Luke is ignoring null MXes. > > On Sat, Feb 25, 2023 at 8:00 PM Benny Pedersen via mailop <mailop@mailop.org > <mailto:mailop@mailop.org>> wrote: >> Luke via mailop skrev den 2023-02-26 04:15: >> > I can assure you sendgrid retries 4xx. We also don't retry 4xx. We >> > also retry 5xx. We also don't retry 5xx. How is it 2023 and people >> > still think 4xx means retry and 5xx means don't retry? >> >> what books do you read ? >> >> maybe rfc 7505 is soon or later for all ? >> _______________________________________________ >> mailop mailing list >> mailop@mailop.org <mailto:mailop@mailop.org> >> https://list.mailop.org/listinfo/mailop > _______________________________________________ > mailop mailing list > mailop@mailop.org > https://list.mailop.org/listinfo/mailop -- The Delivery Experts Laura Atkins Word to the Wise la...@wordtothewise.com Email Delivery Blog: http://wordtothewise.com/blog
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