mx1.etb.net.co has no NS records.
NS records for this zone are defined at etb.net.co.
NS records for this zone are defined at etb.net.co.
Question and response
Question
dig mx1.etb.net.co. NS
Answer
The response has no answer records.
Authority
etb.net.co. 900 SOA ns1-auth.etb.net.co. postmaster.etb.net.co. 1486 10800 3600 2419200 900
Additional
. 0 OPT ; payload 1220, xrcode 0, version 0, flags 0