mx01.provider.nl has no CNAME records.
Question and response
Question
dig mx01.provider.nl. NS
Answer
The response has no answer records.
Authority
provider.nl. 5318 SOA dns.resolver.domains. hostmaster.provider.nl. 2025112701 86400 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0