smtp.cetrel.lu has no CNAME records.
Question and response
Question
dig smtp.cetrel.lu. NS
Answer
The response has no answer records.
Authority
cetrel.lu. 100 SOA ns1.as8677-dns.net. ao-dom.worldline.com. 2023042055 1800 900 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0