The stamps on a received mail should get semi-transparant post marks for each hop from the respective server. Could be generated from the mail header info.
@bleeptrackhttps://anubis.techaro.lol/ same concept, but more widespread in order to complicate web-crawlers' lives. Unfortunately, however, it does not (yet) support dynamic challenge/postmark sizes
Anubis: Web AI Firewall Utility | Anubis
Weigh the soul of incoming HTTP requests to protect your website!