mx.iplanisp.com has no NS records.
NS records for this zone are defined at iplanisp.com.
NS records for this zone are defined at iplanisp.com.
Question and response
Question
dig mx.iplannetworks.net. NS
Answer
The response has no answer records.
Authority
iplannetworks.net. 3600 SOA dns1.iplanisp.com.ar. hostmaster.iplan.com.ar. 2018122806 3600 600 1400000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0