Apex domains cannot have CNAME records.
Question and response
Question
dig mailerhost.net. NS
Answer
mailerhost.net. 5 NS ns-547.awsdns-04.net.
mailerhost.net. 5 NS ns-1275.awsdns-31.org.
mailerhost.net. 5 NS ns-1976.awsdns-55.co.uk.
mailerhost.net. 5 NS ns-288.awsdns-36.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0