If the sender is on the recipient's allow-list then it goes straight through. Otherwise the recipient has to solve a captcha. This allows humans to send small numbers of emails easily, but makes it harder for spammers.

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).