mail.ns1-tec.de has no CNAME records.
Question and response
Question
dig mail.ns1-tec.de. NS
Answer
The response has no answer records.
Authority
ns1-tec.de. 763 SOA ns1-tec.de. webmaster.ns1-tec.de. 2026021801 86400 3600 2592000 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0