mutt.sielsystems.nl has no CNAME records.
Question and response
Question
dig mutt.sielsystems.nl. NS
Answer
The response has no answer records.
Authority
sielsystems.nl. 3600 SOA ns1.sielsystems.nl. beheer.sielsystems.nl. 2026030108 3600 600 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0