I presume it should also be possible to reduce the threshold for my contacts/friends... so that strangers emailing me for the first time need proofs, but frequent contacts don’t.

While reading this, an idea popped up in my head about monetization. Sending a message needs spending a PoW token that can be mined or bought. The price is set by the recipient and can be different for different senders. Zero for friends, a high number for potential spammers. The author, however, has a private key that can produce PoW tokens cheaply, but still in a limited amount to retain trust and prevent diluting everyone's share to nothing. Some companies may want to send spam and will be happy to buy PoW tokens. Users that suddenly start getting spam, raise the bar, as it costs them nothing. Those companies now need more tokens and they come to you. This is inflation. As long as SEC understands what's going on, it shouldn't have problems with this sandbox economics.