Apex domains cannot have CNAME records.
Question and response
Question
dig api-1mail.xyz. NS
Answer
api-1mail.xyz. 5 NS ns75.domaincontrol.com.
api-1mail.xyz. 5 NS ns76.domaincontrol.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0