mx.vtmail.ru has no CNAME records.
Question and response
Question
dig mx.vtmail.ru. NS
Answer
The response has no answer records.
Authority
vtmail.ru. 10721 SOA ns.mts-nn.ru. hostmaster.mts-nn.ru. 2020102701 10800 3600 1500000 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0