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. 302 SOA ns1.mastertel.RU. noc.mastertel.RU. 2026020400 10800 3600 3600000 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0