Unless the IP had a blacklist history that you didn't realize up till now, clearly someone; using your MC installation as a relay, is spamming. Even if they don't think they are sending spam, the recipients surely does. Or sometimes they outright send spam to honeytrap addresses. You better check ab...

- Check if you accidentally became an open relay.
- Check your relayed messages. Maybe one of your domains are sending spam mails. MC only checks for Virus for outgoing mail, not spam.