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Postel’s Law would put the onus on Google to be forgiving in what it receives. Unsure how you could safely use a sender-created Message-Id for anything anyway.

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Following Postel's law results in the normalisation and proliferation of defective implementations. The actual standard becomes irrelevant, and new implementations have to be coded against the defective ones.

My opinion is that Postel's law should be approached in the same way that Linus Torvalds did CVS when designing Git. If in doubt about an implementation decision, consider what Postel's law would recommend, and then do the exact opposite.

That “law” if from a different time, before protocols like SMTP became adversarial. It assumed everyone was acting in good faith.

Yep. And even a world of perfect good faith, "forgiving in what you receive" has both costs and scaling problems - from researching what "spec" you'll need to design to, to customer service when the added complexity and permissiveness cause interesting stuff to happen.

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