mx1.casjay.nl has no CNAME records.
Question and response
Question
dig mx1.casjay.nl. NS
Answer
The response has no answer records.
Authority
casjay.nl. 4212 SOA dns.casjay.nl. dns-admin.casjay.nl. 2026010334 10800 3600 1209600 38400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0