post.th.com has no MX records.
Question and response
Question
dig post.th.com. MX
Answer
The response has no answer records.
Authority
post.th.com. 3600 SOA ns.ksc.co.th. hostmaster.ns.ksc.co.th. 2015111700 43200 300 604800 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0