mx1.fibertel.com.ar has no CNAME records.
Question and response
Question
dig mx1.fibertel.com.ar. NS
Answer
The response has no answer records.
Authority
fibertel.com.ar. 10416 SOA dns1.cvtci.com.ar. noc.fibertel.com.ar. 2026010500 86400 21600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0