smtp3.dtacnetwork.co.th has no MX records.
Question and response
Question
dig smtp3.dtacnetwork.co.th. MX
Answer
The response has no answer records.
Authority
dtacnetwork.co.th. 900 SOA ns5.dtac.co.th. hostmaster.ns5.dtacnetwork.co.th. 2022092273 1800 300 2592000 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0