Apex domains cannot have CNAME records.
Question and response
Question
dig mx1.email. NS
Answer
mx1.email. 5 NS ns-924.awsdns-51.net.
mx1.email. 5 NS ns-49.awsdns-06.com.
mx1.email. 5 NS ns-1685.awsdns-18.co.uk.
mx1.email. 5 NS ns-1299.awsdns-34.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0