mail.o7n.nl has no CNAME records.
Question and response
Question
dig mail.o7n.nl. NS
Answer
The response has no answer records.
Authority
o7n.nl. 133 SOA ns0.transip.net. hostmaster.transip.nl. 2026011600 86400 1800 2419200 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0