mail.sdtnet.de has no CNAME records.
Question and response
Question
dig mail.sdtnet.de. NS
Answer
The response has no answer records.
Authority
sdtnet.de. 921 SOA ns1.sdt.net. hostmaster.sdt.net. 2023041500 28800 7200 2419200 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0