dns4.viettel.com.vn has no MX records.
Question and response
Question
dig dns4.viettel.com.vn. MX
Answer
The response has no answer records.
Authority
viettel.com.vn. 1800 SOA dns1.vietel.com.vn. admin\@vietel.com.vn. 2025103101 10800 3600 604800 1800
Additional
. 0 OPT ; payload 512, xrcode 0, version 0, flags 0