mx1.etb.net.co has no CNAME records.
Question and response
Question
dig mx1.etb.net.co. NS
Answer
The response has no answer records.
Authority
etb.net.co. 223 SOA ns1-auth.etb.net.co. postmaster.etb.net.co. 1486 10800 3600 2419200 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0