Apex domains cannot have CNAME records.
Question and response
Question
dig bullsmta.com. NS
Answer
bullsmta.com. 5 NS ns1.selectel.ru.
bullsmta.com. 5 NS ns2.selectel.ru.
bullsmta.com. 5 NS ns3.selectel.ru.
bullsmta.com. 5 NS ns4.selectel.ru.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0