Apex domains cannot have CNAME records.
Question and response
Question
dig retail.telecomitalia.it. NS
Answer
retail.telecomitalia.it. 5 NS dnsts.interbusiness.it.
retail.telecomitalia.it. 5 NS dnsti.interbusiness.it.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0