smtp.restena.lu has no CNAME records.
Question and response
Question
dig smtp.restena.lu. NS
Answer
The response has no answer records.
Authority
restena.lu. 3600 SOA ns1.restena.lu. noc.restena.lu. 1772923605 14400 1800 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0