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