mx.bit.nl has no CNAME records.
Question and response
Question
dig mx.bit.nl. NS
Answer
The response has no answer records.
Authority
bit.nl. 300 SOA nsauth1.bit.nl. hostmaster.bit.nl. 2026021704 43000 7200 608400 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0