Apex domains cannot have CNAME records.
Question and response
Question
dig mailtelefonica.com.ar. NS
Answer
mailtelefonica.com.ar. 5 NS dns2.advance.com.ar.
mailtelefonica.com.ar. 5 NS dns1.advance.com.ar.
Authority
The response has no authority records.
Additional
dns1.advance.com.ar. 5 A 209.13.119.20
dns2.advance.com.ar. 5 A 209.13.119.21
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0