A few weeks ago, numerous emails from and to many providers could not be delivered or ended up in the famous spam folder. Before sending an e-mail, your provider checks whether the mail sender is on a spam blocking list or not. One of the best known blacklists is Spamcop.net.
Our ComputerButler research showed that the owners of Spamcop.net had simply forgotten to renew or extend their domain. As a result, the site was not accessible and requests from numerous email providers were rejected. The provider checks whether this IP address exists on this blocking list. If you are not on this list, the provider receives an empty DNS response, so everything is fine and you are not a spammer. If a response comes back, you are classified as a spammer. This has also happened to numerous providers using this block list. The systems received no replies to their queries and automatically classified every email sender as a spammer.
Some of these spam blacklists are blocked directly by the provider and you could not send any emails at all. Other providers use this blacklist for spam rating and their emails ended up in the spam folder.