spf.ais.co.th has no MX records.
Question and response
Question
dig spf.ais.co.th. MX
Answer
The response has no answer records.
Authority
spf.ais.co.th. 600 SOA ns3.ais.co.th. please_set_email.absolutely.nowhere. 47 600 600 604800 600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0