Steve Atkins from Word to the Wise explains why it's so important to make sure you're querying a blacklist correctly. Get it wrong, you end up blocking no spam at all, or worse, you end up blocking all of your inbound mail accidentally.
Spam filter authors -- it's time for your software to start rejecting DNSBLs that don't have a properly formatted test record, confirming that they're alive and that the filter in question is properly configured.
