We received many or reports from clients unable to receive email from senders, with our servers reporting that the sending servers are blacklist as spam sources. Turns out it was a false alarm that has since been resolved.
The cause of the problem was (quite embarrassingly) that SpamCop (one of the anti-spam services we use) let their domain registration expire over the weekend. Among other things, they keep an updated list of mail servers that are known to be sending spam: a so-called “blacklist”. When our servers were verifying incoming email against the expired domain of Spam Cop today, things derailed and lots of innocent servers were incorrectly classified as spam sources.
Here is a nice detailed write-up on the problem on The Register.
The issue is resolved and email delivery has returned to normal.
We regret the inconvenience. We are confident that Spam Cop has learned a lesson in the process.