exceeded the maximum nameserver nxdomains

[zebra.squad42 at aceecat.org]

I use unbound on my mail server, among other things it checks for the
presence of connecting IPs in RBL blocklists.

Yesterday I turned on verbose logging including query logs, and today
I see exactly one instance of this:

Jul 15 05:59:26 unbound[20150:0] error: SERVFAIL <11.22.33.44.zen.spamhaus.org. A IN>: exceeded the maximum nameserver nxdomains
Jul 15 05:59:26 unbound[20150:0] reply: 127.0.0.1 11.22.33.44.zen.spamhaus.org. A IN SERVFAIL 0.082448 0 58
Jul 15 05:59:26 unbound[20150:0] query: 127.0.0.1 11.22.33.44.zen.spamhaus.org. A IN
Jul 15 05:59:26 unbound[20150:0] reply: 127.0.0.1 11.22.33.44.zen.spamhaus.org. A IN NXDOMAIN 0.030212 0 122

(The obvious obfuscation has been done.)

What does this mean? I have done a google search but I can't pick out
anything helpful. I am above all making sure I don't violate spamhaus'
fair use policy.

Using unbound 1.17.1 on devuan daedalus.

-- 
Ian