mail.GTNET.RU has no CNAME records.
Question and response
Question
dig mail.GTNET.RU. NS
Answer
The response has no answer records.
Authority
GTNET.RU. 3562 SOA inn361.RU. inn.inn361.ru.GTNET.RU. 2026020902 86400 3600 604800 345600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0