In short: I'm building an email verification service that works via live SMTP probing. One day iCloud started rejecting my server with 550 ... rejected due to listing in Spamhaus — even though my server had never sent a single email.
Source: [Dev.to](https://dev.to/oleggg1/how-spamhaus-blocklisted-my-smtp-verifier-a-server-that-never-sent-an-email-4335)