mx.korean456.com has no CNAME records.
Question and response
Question
dig mx.korean456.com. NS
Answer
The response has no answer records.
Authority
korean456.com. 3510 SOA dns.korean456.com. hostmaster.korean456.com. 1742379088 43200 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0