Apex domains cannot have CNAME records.
Question and response
Question
dig mail.amosrv.ru. NS
Answer
mail.amosrv.ru. 5 NS ns1.amosrv.ru.
mail.amosrv.ru. 5 NS ns2.amosrv.ru.
Authority
The response has no authority records.
Additional
ns1.amosrv.ru. 5 A 88.212.249.4
ns2.amosrv.ru. 5 A 31.41.152.19
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0