mail.mastertel.ru has no CNAME records.
Question and response
Question
dig mail.mastertel.ru. NS
Answer
The response has no answer records.
Authority
mastertel.ru. 2227 SOA ns1.mastertel.ru. noc.mastertel.ru. 2026020400 10800 3600 3600000 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0