Apex domains cannot have CNAME records.
Question and response
Question
dig mailplug.co.kr. NS
Answer
mailplug.co.kr. 4 NS ns.mailplug.com.
mailplug.co.kr. 4 NS ns2.mailplug.com.
mailplug.co.kr. 4 NS ns.mailplug.co.kr.
mailplug.co.kr. 4 NS ns2.mailplug.co.kr.
Authority
The response has no authority records.
Additional
ns.mailplug.co.kr. 4 A 211.253.28.95
ns.mailplug.com. 4 A 14.49.38.75
ns.mailplug.com. 4 A 211.253.28.95
ns.mailplug.com. 4 A 223.26.214.21
ns.mailplug.com. 4 A 223.26.214.40
ns2.mailplug.co.kr. 4 A 14.49.38.179
ns2.mailplug.com. 4 A 14.49.38.179
ns2.mailplug.com. 4 A 211.253.28.172
ns2.mailplug.com. 4 A 223.26.214.22
ns2.mailplug.com. 4 A 223.26.214.41
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0