mx1.hanmail.net has no NS records.
NS records for this zone are defined at hanmail.net.
NS records for this zone are defined at hanmail.net.
Question and response
Question
dig mx1.hanmail.net. NS
Answer
The response has no answer records.
Authority
hanmail.net. 300 SOA ns.kakaocorp.com. root.kakaocorp.com. 1668994734 1800 600 2419200 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0