[networkimprov]

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?"

[LoSboccacc]

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

[networkimprov]

There is no authentication of the John Doe part of

   John Doe <[e-mail]>

[LoSboccacc]

DMARC