I have many questions.
How is this funded? Does the sender pay for each message? Is the payment by size of the message? How is spam prevented? I think I remember POW being recommended to reduce spam. Can we actually find a right balance that doesn't take too long for a legitimate user to solve the proof of work for their outgoing message (especially on a mobile device) but is still hard enough to stop dedicated spammers? Do the mail servers really have an obligation to hold all of these sent messages? What if they drop old messages? In that case, how would a lightweight client know it has received funds?