Apex domains cannot have CNAME records.
Question and response
Question
dig l.antispamcloud.com. NS
Answer
l.antispamcloud.com. 5 NS ns-2012.awsdns-59.co.uk.
l.antispamcloud.com. 5 NS ns-1229.awsdns-25.org.
l.antispamcloud.com. 5 NS ns-4.awsdns-00.com.
l.antispamcloud.com. 5 NS ns-824.awsdns-39.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0