Apex domains cannot have CNAME records.
Question and response
Question
dig custom-email-domain.stripe.com. NS
Answer
custom-email-domain.stripe.com. 5 NS ns-1404.awsdns-47.org.
custom-email-domain.stripe.com. 5 NS ns-994.awsdns-60.net.
custom-email-domain.stripe.com. 5 NS ns-163.awsdns-20.com.
custom-email-domain.stripe.com. 5 NS ns-2008.awsdns-59.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0