Comment by PaulRobinson

"We" tried this in the late 1990s. I remember writing an exim config to do this a couple of ways back in my ISP times:

1. "The recipient will not get this message until you authenticate yourself as a legitimate sender at this URL [...]" - resulted in people never getting mail from no-reply addresses they cared about (banks, e-com, etc.)

2. "This sender has sent you an email, with this subject and this first paragraph. Click here to whitelist, here to blacklist" - created a lot of churn on first setup that users didn't like as a UX

Baking it into the protocol - even now in SMTP and IMAP land - is doable, but would be resisted by email providers who, for example, make their money selling data to marketing agencies. Like, you know, Google, Microsoft, Yahoo...

I've seen other approaches proposed - even at IETF level - over the years, including digital postage stamps (sender pays to send email, your server gets paid, and may reject non-stamped/paying email), but it's the usual network effect: until it becomes widely supported it won't "stick". The stuff that has been adopted has basically been stuff that Google likes, period.