mx.cablevision.net.mx has no CNAME records.
Question and response
Question
dig mx.cablevision.net.mx. NS
Answer
The response has no answer records.
Authority
cablevision.net.mx. 4637 SOA ns02-s01.izzi.mx. fmmartineza.izzi.mx.cablevision.net.mx. 2015073025 21600 900 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0