mailgw2.inet.vn has no CNAME records.
Question and response
Question
dig mailgw2.inet.vn. NS
Answer
The response has no answer records.
Authority
inet.vn. 1800 SOA aiden.ns.cloudflare.com. dns.cloudflare.com. 2398302698 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0