It appears this happened maybe a year ago, but I just found out about it 
this last week as Spamhaus is expanding their blocking of traffic from 
offending hosting companies.

https://www.spamhaus.com/product/data-query-service/

As part of their efforts to keep their public rbl servers available and 
working, they are now putting in IP address blocks in front of their 
traditional spamhaus.org rbl servers. Apparently more than one hosting 
company is in their sights because of abuse of the Spamhaus public 
mirrors and Spamhaus is taking steps to block all IP addresses of the 
offending hosting companies.

This is a simple way to bypass that IP address blocking that Spamhaus is 
implementing.

In the past, it was recommend to NOT use public DNS servers for RBL 
queries, it appears that this could also bypass that restriction.

You sign up for their Data Query Service(DQS) at the link above. It's 
still free for low volume mail servers to us(like me). You just sign up 
for a free account on their system.  You are given a Datafeed Query 
Account Key.  Then in Surgemail(or where ever you use their RBL 
services) you change:

zen.spamhaus.org

to:

<your_DQS_key>.zen.dq.spamhaus.net

That's it.  I just converted my Surgemail server this morning and it's 
working fine. It only takes a few minutes to sign up and change over.

Lyle Giese