mx.tanvoi.com has no CNAME records.
Question and response
Question
dig mx.tanvoi.com. NS
Answer
The response has no answer records.
Authority
tanvoi.com. 3476 SOA dns.tanvoi.com. hostmaster.tanvoi.com. 1299645623 43200 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0