mail.cecosa.com.ar has no CNAME records.
Question and response
Question
dig mail.cecosa.com.ar. NS
Answer
The response has no answer records.
Authority
cecosa.com.ar. 3600 SOA ns303.dnsever.com. root.cecosa.com.ar. 1770828492 86400 600 1209600 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0