After further investigation, it seems SpamCop is producing false positives to all requests as their domain spamcop.net has expired yesterday and wasn't renewed in time.

SpamCop is one of the widely used DNS blacklist providers on the Internet.

Astonishingly, many ISPs seem to solely rely on responses from a single fault-proof source to determine if IPs should be blacklisted and all incoming e-mails blindly rejected.

Hard to believe, but many of your e-mails are currently not getting delivered due to someone forgetting to renew their domain, which so much of our personal and business functionality apparently relies on.