[arkaine]

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/

[altharaz]

From my experience, it seems that the daily digest is not enough :). And they still seem to blacklist WebEx invitations, which is really weird as it is definitely a "standard" in web-conferences.