mail.online.net has no CNAME records.
Question and response
Question
dig mail.online.net. NS
Answer
The response has no answer records.
Authority
online.net. 10800 SOA nsa.online.net. dnsmaster.te-dns.net. 2026030501 14400 3600 604800 14400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0