mails.online.com.kh has no CNAME records.
Question and response
Question
dig mails.online.com.kh. NS
Answer
The response has no answer records.
Authority
online.com.kh. 2015 SOA dns1.online.com.kh. hostmaster.online.com.kh. 2026022502 10800 3600 43200 43200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0