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