_spf.thnic.co.th has no MX records.
Question and response
Question
dig _spf.thnic.co.th. MX
Answer
The response has no answer records.
Authority
thnic.co.th. 180 SOA a-ns.thnic.co.th. admin.thnic.co.th. 2602250051 21600 900 1252800 7200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0