Apex domains cannot have CNAME records.
Question and response
Question
dig mailman4.com. NS
Answer
mailman4.com. 5 NS ns-368.awsdns-46.com.
mailman4.com. 5 NS ns-1597.awsdns-07.co.uk.
mailman4.com. 5 NS ns-1493.awsdns-58.org.
mailman4.com. 5 NS ns-799.awsdns-35.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0