mx.telecom.com.ar has no CNAME records.
Question and response
Question
dig mx.telecom.com.ar. NS
Answer
The response has no answer records.
Authority
telecom.com.ar. 63 SOA jazzy.arnet.com.ar. dnsmaster.arnet.net.ar. 200911404 7200 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0