getUserByIdがユーザーのメールを返さないのはなぜですか？

[Canapin (Coin-coin le Canapin)]

It doesn’t exactly reply to your question, but does this help?

Is there a way to get a user email adress from the API?

GET /users/[username]/emails.json

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Probably of no use for your case since you can't dynamically change the user's ID as far as I know

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