Apex domains cannot have CNAME records.
Question and response
Question
dig deliver.cloudcone.email. NS
Answer
deliver.cloudcone.email. 5 NS ac1.cloudcone.net.
deliver.cloudcone.email. 5 NS ac2.cloudcone.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0