Apex domains cannot have CNAME records.
Question and response
Question
dig freshemail.io. NS
Answer
freshemail.io. 5 NS ns-519.awsdns-00.net.
freshemail.io. 5 NS ns-1972.awsdns-54.co.uk.
freshemail.io. 5 NS ns-285.awsdns-35.com.
freshemail.io. 5 NS ns-1213.awsdns-23.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0