dns-master.vnnic.vn has no MX records.
Question and response
Question
dig dns-master.vnnic.vn. MX
Answer
The response has no answer records.
Authority
vnnic.vn. 5400 SOA dns1.vnnic.vn. postmaster.vnnic.vn. 2023035713 1800 900 604800 5400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0