Apex domains cannot have CNAME records.
Question and response
Question
dig squarespace-mail.com. NS
Answer
squarespace-mail.com. 5 NS ns04.squarespacedns.com.
squarespace-mail.com. 5 NS ns03.squarespacedns.com.
squarespace-mail.com. 5 NS dns1.p02.nsone.net.
squarespace-mail.com. 5 NS ns02.squarespacedns.com.
squarespace-mail.com. 5 NS dns3.p02.nsone.net.
squarespace-mail.com. 5 NS ns01.squarespacedns.com.
squarespace-mail.com. 5 NS dns4.p02.nsone.net.
squarespace-mail.com. 5 NS dns2.p02.nsone.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0