Apex domains cannot have CNAME records.
Question and response
Question
dig servicioatt.com.mx. NS
Answer
servicioatt.com.mx. 4 NS mail2.alestra.net.mx.
servicioatt.com.mx. 4 NS mail1.alestra.net.mx.
servicioatt.com.mx. 4 NS dns.alestra.net.mx.
Authority
The response has no authority records.
Additional
dns.alestra.net.mx. 4 A 201.163.57.5
mail1.alestra.net.mx. 4 A 201.163.56.5
mail2.alestra.net.mx. 4 A 201.163.58.5
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0