ptr.twnic.net.tw has no MX records.
Question and response
Question
dig ptr.twnic.net.tw. MX
Answer
The response has no answer records.
Authority
ptr.twnic.net.tw. 600 SOA ptr1.twnic.net.tw.ptr.twnic.net.tw. tech.twnic.net.tw.ptr.twnic.net.tw. 7 86400 3600 604800 10800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0