Also, recipients could have deny-lists and automatically share them with others they trust. The deny-lists might be a list of senders' cryptographically generated IDs, in which case it should be reasonably hard to generate new ones (e.g. 10 minutes on an X86 cpu using an algorithm not conducive to parallelisation or special hardware).
No way am I going to solve captchas for incoming emails! Not any.
Yeah mining coins totally doesn't exclude poor people. If it costs less than the profit spammers make they will use it. You can't just handwave the thresold away, and pretend you are solving the problem by just asking for money(thereby excluding large swaths of people). Just use your iMessage.
That said, since this problem already exists in the world, it is often overstated.
If a stamp is a reasonable cost, and only a one time cost, it will be largely invisible. 25 cents (a price set by the recipient) to send a message from a stranger won't have a big impact on most people, and it's important to recognize it's only the first time.
It's further important to know it would even be possible to refund the cost of the stamp back to the sender once the message is sent. The recipient could have a button that says "Return stamp" which would then let the sender re-use or in some other way organize the stamp.
Now let's contrast to today... Let's say I want to send a message to a big movie star or a CEO. The chances that I will be able to get their address is relatively low. Even if I do, they likely have someone they pay to filter their messages before it gets to them.