My friend Mickey Chandler pointed out recently that he's been seeing some unusual bounces that look like this:

Host blacklisted - Found on Realtime Black List server

It turns out that you block all of your inbound mail with an error like this if you configure "" in your mail server as a DNSBL zone for blocking purposes.

Why? Because there is no such zone as Looks like Spamhaus set up these DNS responses to queries of this non-existent zone so that people would quickly realize that they're querying the wrong thing.

Actual Spamhaus DNSBL zones include,,, and others. I recommend using them, the ZEN zone in particular, as they're very accurate. But, regardless of what zone you use, it's important to use one that actually exists.

Users of will find no spam blocking value from use of this zone, and are likely to find all of their inbound mail rejected.