ns2.mailplug.co.kr has no MX records.
Question and response
Question
dig ns2.mailplug.co.kr. MX
Answer
The response has no answer records.
Authority
mailplug.co.kr. 400 SOA ns.mailplug.com. root.mailplug.com. 2026010200 21600 1800 1209600 400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0