Apex domains cannot have CNAME records.
Question and response
Question
dig smtp.masterhost.ru. NS
Answer
smtp.masterhost.ru. 5 NS ns2.masterhost.ru.
smtp.masterhost.ru. 5 NS ns1.masterhost.ru.
smtp.masterhost.ru. 5 NS ns.masterhost.ru.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0