mail.mastertel.com has no CNAME records.
Question and response
Question
dig mail.mastertel.com. NS
Answer
The response has no answer records.
Authority
mastertel.com. 3569 SOA ns1.mastertel.ru. noc.mastertel.ru. 2025101501 10800 3600 3600000 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0