Apex domains cannot have CNAME records.
Question and response
Question
dig mbox.contact.bg. NS
Answer
mbox.contact.bg. 5 NS ns3.contact.bg.
mbox.contact.bg. 5 NS sns-pw.contact.bg.
mbox.contact.bg. 5 NS pns-pw.contact.bg.
mbox.contact.bg. 5 NS ns4.contact.bg.
Authority
The response has no authority records.
Additional
ns3.contact.bg. 5 A 212.39.88.81
ns4.contact.bg. 5 A 212.39.80.137
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0