mail.sisytec.net has no CNAME records.
Question and response
Question
dig mail.sisytec.net. NS
Answer
The response has no answer records.
Authority
sisytec.net. 7524 SOA ns1.linode.com. sistemas.sisytec.net. 2021000118 14400 14400 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0