Apex domains cannot have CNAME records.
Question and response
Question
dig flockmail.com. NS
Answer
flockmail.com. 5 NS ns-973.awsdns-57.net.
flockmail.com. 5 NS ns-299.awsdns-37.com.
flockmail.com. 5 NS ns-1609.awsdns-09.co.uk.
flockmail.com. 5 NS ns-1400.awsdns-47.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0