> Small cost per email like 0.01 cents that is trivial for legitimate senders but not for bulk-sending spammers
Given how much is spent on ads, I’d say that an email is still worth more than that. So, what you get is spammers paying minuscole amount of money to skip the spam filter and killing the idea at the root.
Or they can simply steal these credentials from one of the millions of hacked sites and cause additional trouble to them.
Within the confines of "a digital stamp is a good idea" [1], what you're saying isn't quite true: instead of being treated as a whitelist, a digital stamp can just be seen as an finite increase in credibility of some quantum that's decided over time by adaptive filters.
Given how much is spent on ads, I’d say that an email is still worth more than that. So, what you get is spammers paying minuscole amount of money to skip the spam filter and killing the idea at the root.
Or they can simply steal these credentials from one of the millions of hacked sites and cause additional trouble to them.