Apex domains cannot have CNAME records.
Question and response
Question
dig RUSENDER-MAIL-100.RU. NS
Answer
RUSENDER-MAIL-100.RU. 4 NS ns1.domenus.RU.
RUSENDER-MAIL-100.RU. 4 NS ns2.domenus.RU.
Authority
The response has no authority records.
Additional
ns1.domenus.RU. 4 A 77.87.208.8
ns2.domenus.RU. 4 A 31.28.20.107
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0