This proposal seems to make the assumption that senders need their incentives adjusted - you need to pay some amount to send someone a message.

I can't see anything that stops a perverse incentive being created on the recipient's part: Why wouldn't I create a harvesting mailbox which signs up for as many messages as it can to generate income?

It seems like there would need to be a feedback mechanism so that senders could tell who was actually a recipient worth paying to talk to.