Spamhaus has a fair use policy which prohibits sending queries from open resolvers or sending queries in large volumes. To indicate an error, they send back specific IP addresses. Since we aren't expecting these we end up treating  it as a hit on their blacklist.

More info: https://www.spamhaus.com/product/help-for-spamhaus-public-mirror-users/

To test, you can set your resolver to 1.1.1.1 or 9.9.9.9 and attempt to post with DNSBL checking enabled.