Great, another cost to making free software. As if building installers for difficult platforms and moving targets (macOS) and dealing with antivirus software (code signing certs cost $$) wasn't enough, now charge me for sending a newsletter. Everyone is a company, right?

Edit, having read the article: If you're going to put that much cryptography into it, why not give a unique key when you subscribe? That way you can unsubscribe or reduce volume at any point, by dropping messages with that key (and all messages without keys). As usual, what is the extra blockchain for?