Mailinblack does this for more than ten years now, mostly in France. This technique is called a "challenge-response" filtering. It does work pretty well with ot

Mailinblack does this for more than ten years now, mostly in France. This technique is called a "challenge-response" filtering. It does work pretty well with other antispam techniques. Their solution send a daily digest to the user (x per day) of quarantined emails, allowing them to liberate legitimate emails of lazy senders who do not respond to the challenge-response email. They also use outgoing emails from users to populate their personal whitelist. Their site is not shinny but the product does the job. https://www.mailinblack.com/en/ https://www.mailinblack.com/en/