Since the article relates to email and mentions digital stamps, it seems worth mentioning Microsoft's Penny Black[1] anti-spam research project which had/has so

Since the article relates to email and mentions digital stamps, it seems worth mentioning Microsoft's Penny Black[1] anti-spam research project which had/has some similar goals. In general the idea (from both the linked article and the research project) is to make it computationally expensive to send messages, in a way that is cheap for the recipient to verify. That reduces both the desire and the capability for scammers to widely distribute their messages. [1] - https://en.wikipedia.org/wiki/Penny_Black_(research_project) https://en.wikipedia.org/wiki/Penny_Black_(research_project)