mail.mirae.co.kr has no MX records.
Question and response
Question
dig mail.mirae.co.kr. MX
Answer
The response has no answer records.
Authority
mirae.co.kr. 600 SOA mirae.co.kr. root.mirae.co.kr. 2011122101 3600 1200 864000 600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0