Apex domains cannot have CNAME records.
Question and response
Question
dig submission.antispamcloud.com. NS
Answer
submission.antispamcloud.com. 5 NS ns-1242.awsdns-27.org.
submission.antispamcloud.com. 5 NS ns-996.awsdns-60.net.
submission.antispamcloud.com. 5 NS ns-200.awsdns-25.com.
submission.antispamcloud.com. 5 NS ns-1885.awsdns-43.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0