Apex domains cannot have CNAME records.
Question and response
Question
dig mail.tiscali.it. NS
Answer
mail.tiscali.it. 5 NS murdock.dns.tiscali.it.
mail.tiscali.it. 5 NS hannibal.dns.tiscalinet.it.
mail.tiscali.it. 5 NS barakus.dns.tiscali.it.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0