Like cryptocurrency this will be a moving target. You would need a difficulty setting - but where as cryptocurrency only has to track one thing "total hash rate" this would need to track two things "minimum supported hardware for legit sender" and "hardware threshold for attacker to be successful with a mining rig they can afford, based on the income from the spam".
Each email provider might come up with different difficulty levels based on what they thing this is. So some handshaking might be required. And less computer literate people would be stressed why their email is taking 6 minutes to send. I think it would be hard to implement.
