> "In a spammer’s economic model, spending even five or 10 seconds per message could be prohibitively expensive" So, how about a proof of work system like bloc

> "In a spammer’s economic model, spending even five or 10 seconds per message could be prohibitively expensive" So, how about a proof of work system like blockchains? Like, if you've never sent me email before, my server returns some work to complete. If I've ever sent you an email, or not marked you mail as spam etc, your MTA won't be challenged etc.