mx.tgos.de has no CNAME records.
Question and response
Question
dig mx.tgos.de. NS
Answer
The response has no answer records.
Authority
tgos.de. 839 SOA ns1.plusserver.com. hostmaster.plusserver.com. 2025041105 1800 900 604800 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0