b.ns.mailus.de has no CNAME records.
Question and response
Question
dig b.ns.mailus.de. NS
Answer
The response has no answer records.
Authority
mailus.de. 10800 SOA a.ns.mailus.de. dns.regfish-ns.net. 1712920357 28800 7200 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0