Apex domains cannot have CNAME records.
Question and response
Question
dig b2b-mail.net. NS
Answer
b2b-mail.net. 5 NS ns-1878.awsdns-42.co.uk.
b2b-mail.net. 5 NS ns-860.awsdns-43.net.
b2b-mail.net. 5 NS ns-101.awsdns-12.com.
b2b-mail.net. 5 NS ns-1470.awsdns-55.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0