Feasibility of idea to reduce email spam

Had an idea for a new email protocol to reduce email spam. It'd be great if someone can tell me if this has been thought of before, and/or identify any reason why this would fail.

Process to send an email would be as follows:

• Sender initiates new email request to recipient


• Recipient returns long random list of integers (the problem)


• Sender must send back ordered list of integers along with email


• Recipient rejects email if integers are not ordered correctly

The aim is to make sender have to solve some sort of computationally expensive problem which is computationally cheap for the recipient to validate (there's probably better problems than just sorting lists).

Recipients can easily scale the complexity of problem over time or to their specific needs (1,000, 20,000, 100,000 list items for example).

"Cold emailing" thousands of people has now become too expensive/time consuming to be viable for the majority of spammers.