[sparkie]

Advertisers will still pay, but they would need to adjust their strategy to target their ads more carefully to those who are likely to result in sales.

For legitimate senders, the attached fee would end up being spent by the receiver to send a reply, like a "refund", so it ends up zero-sum.

But for spammers it becomes an expensive option. Nobody is going to reply to the spam to "refund" their fee.