mx01.exofilter.com has no CNAME records.
Question and response
Question
dig mx01.exofilter.com. NS
Answer
The response has no answer records.
Authority
exofilter.com. 3397 SOA ns1.exonet.nl. hostmaster.exonet.nl. 2026022601 28800 7200 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0