You can even do that, plus add two things. 1) mail that gets filtered gets an automatic reply 'your email went into the spam folder, here's how to reach me if i

You can even do that, plus add two things. 1) mail that gets filtered gets an automatic reply 'your email went into the spam folder, here's how to reach me if it's genuine', and 2) any email with say $1 or some threshold you can manually set is accepted into the inbox with a 'paid' label. Things like bitcoin have made it pretty easy to add digital money attachment to email as an anti-spam measure, first proposed as a proof of work system by Cryptographer Adam Back to introduce a cost to sending email, a bit like a computerised form of a captcha, something that doesn't affect normal users but is disastrously expensive for power users like spammers. The combination ensures that 1) spam gets filtered 2) genuine emailers flagged as spammers get notified their email didn't arrive 3) important email can reach the recipient if it's worth enough (say $1, totally undoable for a spammer who'd end up with a cost-per-click on his email campaign of hundreds of dollars instead of cents, completely unworkable on any scale, but financially no problem for when say a stranger wants to send you a genuine email for the first time with a non-trusted key).