Apex domains cannot have CNAME records.
Question and response
Question
dig api-mail.com. NS
Answer
api-mail.com. 3 NS ns73.domaincontrol.com.
api-mail.com. 3 NS ns74.domaincontrol.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0