I would imagine a system with a whitelist. Anyone not on the whitelist, gets an automatic reply and his email is not even processed. The reply email has a captc

I would imagine a system with a whitelist. Anyone not on the whitelist, gets an automatic reply and his email is not even processed. The reply email has a captcha, once you solve it you can send the email that will be processed and read. The receiver can decide whether to put you on a whitelist or not. So a friendly message will require at most a single captcha, and from then on work normally. Those emails that get trough, have shown enough effort for you to read, and if it is spam, you put it on an always-block-blacklist. The captcha or something equivalent has to be only hard enough so the spammers time isn't worth it. Of course email replies to you would always get trough.