a.dns-servers.vn has no MX records.
Question and response
Question
dig a.dns-servers.vn. MX
Answer
The response has no answer records.
Authority
dns-servers.vn. 5400 SOA dns-master.vnnic.vn. postmaster.vnnic.vn. 2017014414 3600 1800 604800 5400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0