Apex domains cannot have CNAME records.
Question and response
Question
dig onesignal.email. NS
Answer
onesignal.email. 5 NS gigi.ns.cloudflare.com.
onesignal.email. 5 NS ashton.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0