Apex domains cannot have CNAME records.
Question and response
Question
dig mail.cloud.rt.ru. NS
Answer
mail.cloud.rt.ru. 5 NS ns6.o7.com.
mail.cloud.rt.ru. 5 NS ns5.o7.com.
Authority
The response has no authority records.
Additional
ns5.o7.com. 4 A 46.61.230.214
ns6.o7.com. 4 A 81.177.99.16
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0