Interesting ... it's more or less like the 90s call rates or post stamp fee. Maybe the most effective way to combat spam is to "tax" it? I would gladly pay 10 cent to the called or receiver for every call or mail I sent to recipients who hadn't whitelisted me, as long as it was mutual. The spammers would not.

Then again, I still receive alot of paper spam in my physical mailbox and humans paid full time to sell me stuff still call where the call cost is not the main cost, so it wouldn't solve very much and would be hard to organize practically.