mail.GAZTELECOM.RU has no CNAME records.
Question and response
Question
dig mail.GAZTELECOM.RU. NS
Answer
The response has no answer records.
Authority
gaztelecom.ru. 3600 SOA ns.gaztelecom.ru. root.gaztelecom.ru. 2017081701 10800 3600 1209600 38400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0