Apex domains cannot have CNAME records.
Question and response
Question
dig c5mail.net. NS
Answer
c5mail.net. 4 NS ns-326.awsdns-40.com.
c5mail.net. 4 NS ns-1216.awsdns-24.org.
c5mail.net. 4 NS ns-836.awsdns-40.net.
c5mail.net. 4 NS ns-1825.awsdns-36.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0