I don't think the current firehose model of spam would be sustainable anymore, though. Those spammers send millions of mails a day. Even with a 1 cent cost, they'd have to be much more selective about their address lists, given the low success rate. It may not solve the problem but I'm almost sure it would help a little. It also may be an additional qualitative barrier for crime-linked spam such as phishing mails, because they'd have to try and find a non-traceable was of payment, which is not trivial and always carries a slight risk of being identified anyway.

Email2000 is the only answer: https://cr.yp.to/im2000.html

TLDR: Mail storage is the sender's responsibility. The message isn't copied to the receiver. All the receiver needs is a brief notification that a message is available.

I read about an idea for an incentive/check system like that before. Something like: make the cost 10c instead of 1c, but implement a system where recipients can mark mails as confirmed "wanted" mail, upon which the sender would be reimbursed 9c. Increasing the cost for unsolicited mails while keeping the cost low for well-behaved newsletters.