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Comment by networkimprov | Hacker News Reader

You can (sort of, with a great deal of effort) but you don't.

And there is no effective way to prevent phishing in SMTP/etc if the server accepts connections from the public Internet.

If you didn't, I suggest reading the protocol draft and "Why TMTP?"

you can check that the envelop and message sender header match at the mx level and let spf handle the rest

There is no authentication of the John Doe part of

   John Doe <not.john.doe@example.com>

DMARC