mf.onlinemail.io has no CNAME records.
Question and response
Question
dig mf.onlinemail.io. NS
Answer
The response has no answer records.
Authority
onlinemail.io. 300 SOA ns1.curanet.dk. hostmaster.curanet.dk. 2025081201 14400 3600 1209600 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0