Re-thinking electronic mail (2022)
liw.fi
liw.fi
As one example, consider the proposed "recipient stamps". It requires a complex trust configuration, always-on email clients, and synchronizing 'stamp books' across different clients. I expect that makes it intractable in practice.
Better would be 'postage' where each email contains references to one or more payment methods. The receiver and intermediaries can then deduct a fee from any available payment methods at their discretion. Some common postage methods would be a hashcash service, or crypto-currency address, or pre-approved captcha credits, or a service to request a moderately large Internet transfer.
And since the receiver can choose to not cash in the postage, the postage need not be validated at all.