Apex domains cannot have CNAME records.
Question and response
Question
dig emailservice.co. NS
Answer
emailservice.co. 5 NS ns-1651.awsdns-14.co.uk.
emailservice.co. 5 NS ns-1081.awsdns-07.org.
emailservice.co. 5 NS ns-219.awsdns-27.com.
emailservice.co. 5 NS ns-938.awsdns-53.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0