Good point! Indeed HashCash is very similar. And probably where I actually got the main idea, as I'm absolutely certain I read about HashCash long ago. Seems th

Good point! Indeed HashCash is very similar. And probably where I actually got the main idea, as I'm absolutely certain I read about HashCash long ago. Seems that the idea was left to ferment in my head after all these years. One difference is a more interactive proof-of-work procedure where the receiver controls the amount of sender work required; with a zero amount of work (for trusted senders such as mailing lists and known people), the communication would basically behave like e-mail does today. Something like HashCash seems to me like a good idea in general. I don't know why it didn't really take off.