mail3.dotnetpia.co.kr has no MX records.
Question and response
Question
dig mail3.dotnetpia.co.kr. MX
Answer
The response has no answer records.
Authority
dotnetpia.co.kr. 3600 SOA ns1.namesvr.net. root.dotnetpia.co.kr. 2020010610 3600 300 604800 3600
Additional
. 0 OPT ; payload 1680, xrcode 0, version 0, flags 0