Apex domains cannot have CNAME records.
Question and response
Question
dig fastwebmail.com. NS
Answer
fastwebmail.com. 5 NS ns-1883.awsdns-43.co.uk.
fastwebmail.com. 5 NS ns-26.awsdns-03.com.
fastwebmail.com. 5 NS ns-745.awsdns-29.net.
fastwebmail.com. 5 NS ns-1157.awsdns-16.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0