Apex domains cannot have CNAME records.
Question and response
Question
dig mx.smartmail.com.ar. NS
Answer
mx.smartmail.com.ar. 2 NS ns1.allytech.com.
mx.smartmail.com.ar. 2 NS ns2.allytech.com.
Authority
The response has no authority records.
Additional
ns1.allytech.com. 1 A 200.68.105.101
ns2.allytech.com. 1 A 200.68.105.102
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0