mailserver.mostcom.ru has no CNAME records.
Question and response
Question
dig mailserver.mostcom.ru. NS
Answer
The response has no answer records.
Authority
mostcom.ru. 1438 SOA ns.mostcom.ru. hostmaster.mostcom.ru. 2024011404 10800 1800 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0