mailserver.pavietnam.com has no CNAME records.
Question and response
Question
dig mailserver.pavietnam.com. NS
Answer
The response has no answer records.
Authority
pavietnam.com. 3600 SOA dns1.pavietnam.com. admin.pavietnam.vn. 2026013000 3600 1800 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0