There was a post a while back from someone not receiving mail, related to a Spamhaus something.
I can’t find the specific thread 1-2-3, but searching for Spamhaus on the forum returns this thread by Maniack_Crudelis.
How Spamhaus determines which DNS resolver is being used, and at which level: no idea (but I do after edit2)
- edit - I found the other thread; Aleks suggests it has to do with ports 53/5353 being open on our own server (which would allow DNS amplification attacks, I think, but I wonder why Spamhaus would monitor that). There does not seem to be a resolution in that specific thread.
- edit2 - You know how to use search engines, but that way I found a blogpost delving into the problem, with a link to a technical explanation of DNSBL, which is where the open resolver comes in.