Lifthrasil: The idea of solving spam in a financial way is old - and has made some waves many years in the past. When e-mail spam was still a problem, Bill Gates suggested to make e-mails cost something. Not a lot, something like 0.1 cent per mail. Something that wouldn't really affect private mailing, but which would hurt spammers, who send out millions of automated mails per day.
There was a huge outcry: "Bill Gates wants to restrict the freedom of the internet! He wants to squeeze even more money out of us! Outrage!" ... even though he didn't suggest that Microsoft would be getting those 0.1 cents per mail. But it was seen as an attack on the internet and was booed down.
I dont know anything about this but my general POV on it is that there is no sane reason which I can see why they wouldnt add a "this cost me 0.1 cent" system to Email. It would be trivial (from a society POV) to do it and I cant see any disadvantages. Example:
Make a cryptocurrency (or simply use an existing one) which has addresses which belong to no one and cant be used by anybody, bitcoin for example has such.
Optionally add to Emails (in the header or in the body, can be done manually or automatically) a code which can be used to decrypt a comment to a cryptocurrency transfer to such an address. This (adding this or not) is decided by the sender.
The receiver can then set their Email program or provider to disallow Emails without proof-of-money-wasted comment. This would be just one piece in the Email filtering which is possible anyway. They can also set a minimum-money-wasted limit if they want. If an Email is rejected due to this the provider/program will return it to the sender with the respective explanation so that the sender knows it was not received and why it was not received.
From a users perspective this would be very simple to use. Copy a code to the Email or let the program/provider automatically do it after setting it up. The same codes could also be used for other authentication stuff which uses captcha now.
Edit: Missing piece: Obviously the receiver needs to invalidate the codes by uploading them to the cryptocurrency chain or sth similar.
Edit2: Setting it up would also be (potentially) trivial. Just send an Email (containing a list of codes) to an Email address and the Email program/provider would add these codes to the account. So as end user the only effort is to have somebody send me a list of codes to my Email address and the rest would work automagically. Assuming that I (or that 'somebody') has enough money to trivialize the 0.0007 USD cost per code (or whatever the exact amount would be).