Cisco’s SpamCop anti-spam service suffered an outage Sunday after its domain was mistakenly allowed to expire. The service provides a Real-time Blackhole List (RBL) to determine if incoming mail should be considered spam. Cisco is aware of the issue and has been working on renewing the spamcop.net domain throughout the day. As a response was received by the incoming mail server, it would incorrectly block the email as if it was from a known spammer. As of Monday morning, admins have told BleepingComputer that the service is working correctly again.
Source: https://www.bleepingcomputer.com/news/security/spamcop-anti-spam-service-suffers-an-outage-after-its-domain-expired/