"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.
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.