Apex domains cannot have CNAME records.
Question and response
Question
dig transactional-mail-d.com. NS
Answer
transactional-mail-d.com. 5 NS ns-1275.awsdns-31.org.
transactional-mail-d.com. 5 NS ns-924.awsdns-51.net.
transactional-mail-d.com. 5 NS ns-416.awsdns-52.com.
transactional-mail-d.com. 5 NS ns-1552.awsdns-02.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0