Apex domains cannot have CNAME records.
Question and response
Question
dig transactional-mail-a.com. NS
Answer
transactional-mail-a.com. 5 NS ns-612.awsdns-12.net.
transactional-mail-a.com. 5 NS ns-1383.awsdns-44.org.
transactional-mail-a.com. 5 NS ns-1704.awsdns-21.co.uk.
transactional-mail-a.com. 5 NS ns-264.awsdns-33.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0