mx01.whitelabelserver.nl has no CNAME records.
Question and response
Question
dig mx01.whitelabelserver.nl. NS
Answer
The response has no answer records.
Authority
whitelabelserver.nl. 932 SOA ns1.whitelabelserver.nl. hostmaster.whitelabelserver.nl. 2026021200 3600 3600 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0