mail.domserver.de has no CNAME records.
Question and response
Question
dig mail.domserver.de. NS
Answer
The response has no answer records.
Authority
domserver.de. 10752 SOA ns15.domserver.de. oo.gerwan.de. 2024040503 40000 7200 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0