Apex domains cannot have CNAME records.
Question and response
Question
dig pec.ad.aruba.it. NS
Answer
pec.ad.aruba.it. 5 NS dcdmz1.ad.aruba.it.
pec.ad.aruba.it. 5 NS dc1.ad.aruba.it.
pec.ad.aruba.it. 5 NS addns01.ad.aruba.it.
pec.ad.aruba.it. 5 NS pecbackup.pec.ad.aruba.it.
pec.ad.aruba.it. 5 NS dc2.ad.aruba.it.
pec.ad.aruba.it. 5 NS addns02.ad.aruba.it.
pec.ad.aruba.it. 5 NS dcdmz2.ad.aruba.it.
pec.ad.aruba.it. 5 NS it3dc1.ad.aruba.it.
pec.ad.aruba.it. 5 NS adpec02.pec.ad.aruba.it.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0