mx.tviinet.ru has no CNAME records.
Question and response
Question
dig mx.tviinet.ru. NS
Answer
The response has no answer records.
Authority
tviinet.ru. 138 SOA ns1.tviinet.ru. root.tviinet.ru. 2025051335 3600 600 360000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0