This is an interesting service which is a bit like Hey.com’s “screener” feature (require allow-listing for all new email correspondents). Basically it sends everyone an auto response the first time they message you, asking you to prove you’re human and that the email is worth your time.
Reminds me of “proof of work” in bitcoin currencies. A downside is that it requires you to forward mail through their servers (privacy / security) and costs €€. Still, cool concept that keeps your inbox cleaner (and annoys your old school friends and new business contacts): Products MAILINBLACK PROTECT - Mailinblack
Thanks to Jos Poortvliet for the tip