mail.selbstdenker.com has no CNAME records.
Question and response
Question
dig mail.selbstdenker.com. NS
Answer
The response has no answer records.
Authority
selbstdenker.com. 300 SOA anycast.regdns1.de. root.netzblick.de. 2026030301 10800 3600 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0