I did a sort of PoS proof of concept for spam mitigation. The idea was a small refundable fee would be defined at an inbox-level on a distributed ledger. If a recipient flagged content as offensive or spam, the gas fee required to transmit the message was retained, otherwise, it was returned, such that legitimate mail was free to send and spam was expensive.
While the fees were outstanding prior to receipt (or rejected by the recipient), they could participate in interest-earning liquidity pools to fund global efforts, like planting trees.
While the fees were outstanding prior to receipt (or rejected by the recipient), they could participate in interest-earning liquidity pools to fund global efforts, like planting trees.