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…lots of major mail services do?

https://www.fastmail.help/hc/en-us/articles/360060591413-Spa...

https://senders.yahooinc.com/smtp-error-codes/

https://www.xfinity.com/support/articles/email-errors

Honestly those are used as weights now in some kind of obscure calculation (or maybe not so obscure calculation but I still don't understand it).

What I really learned during that time is that mail servers have well known IP addresses and reputations. And you can say all you want SPF/DKIM fixes that, but the reality is when google sends your email (even with your custom domain) it gets received.

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