Apex domains cannot have CNAME records.
Question and response
Question
dig emailservice.io. NS
Answer
emailservice.io. 5 NS ns-160.awsdns-20.com.
emailservice.io. 5 NS ns-2020.awsdns-60.co.uk.
emailservice.io. 5 NS ns-1396.awsdns-46.org.
emailservice.io. 5 NS ns-541.awsdns-03.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0