Apex domains cannot have CNAME records.
Question and response
Question
dig freshmail.io. NS
Answer
freshmail.io. 5 NS ns1.emailverification.info.
freshmail.io. 5 NS ns2.emailverification.info.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0