I don't see why you are getting downvoted. Proof of work is a great way to limit spam. It could be made almost unnoticeable by the end user sending a couple of

I don't see why you are getting downvoted. Proof of work is a great way to limit spam. It could be made almost unnoticeable by the end user sending a couple of messages, but expensive for spam. There are of course some show stoppers (mailing list, non-profits etc), but it is not a bad solution.