mail.de-nserver.de has no CNAME records.
Question and response
Question
dig mail.de-nserver.de. NS
Answer
The response has no answer records.
Authority
de-nserver.de. 145 SOA ns1.de-nserver.de. hostmaster.ns1.de-nserver.de. 2026022723 14400 3600 604800 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0