Perhaps just a router config issue?

Router is Billion ADSL router.

My ISP (3BB) certainly doesnt. Nor does Sophon.

I've certainly come across ISPs (not in Thailand) who block access to outgoing mail servers that are not their own and also prevent users operating their own mail server at home (citing spam prevention).

Not saying this is our OPs issue, just that it does happen.

It's looking like a lot of people are using SpamHaus blacklists that include may ToT IP ranges.