mx.fibercorp.com.ar has no CNAME records.
Question and response
Question
dig mx.fibercorp.com.ar. NS
Answer
The response has no answer records.
Authority
fibercorp.com.ar. 349 SOA o200.prima.com.ar. dnsmaster.prima.com.ar. 2025111805 86400 21640 604800 259200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0