Apex domains cannot have CNAME records.
Question and response
Question
dig m.smailru.net. NS
Answer
m.smailru.net. 5 NS ns2.mail.ru.
Authority
The response has no authority records.
Additional
ns2.mail.ru. 5 A 94.100.180.138
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0