Apex domains cannot have CNAME records.
Question and response
Question
dig purelymail.com. NS
Answer
purelymail.com. 5 NS ns-2013.awsdns-59.co.uk.
purelymail.com. 5 NS ns-373.awsdns-46.com.
purelymail.com. 5 NS ns-990.awsdns-59.net.
purelymail.com. 5 NS ns-1203.awsdns-22.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0