I have to default someone before a certain date and this should happen by registered mail as stated in the agreement.

In Thailand there are two ways for registered mail which are Signed on delivery and EMS, are both accepted as a legal instrument ?

What if the recipient refuse to collect the letter ? In my country a registered letter is assumed to be legally delivered 3 days after it was presented for the first time by the postman. Is this law also available in Thailand?

Most lawyers would send an ems, but a 'street wise' recipient would not sign for it which leaves you in a legal hole. Welcome to the deficient Thai legal system.

So a law as in most countries exist, in that a refused letter is considered to be delivered on the third day, doesn't exist in Thailand ?

How I can be assured that it will be legally delivered?

So a law as in most countries exist, in that a refused letter is considered to be delivered on the third day, doesn't exist in Thailand ?

How I can be assured that it will be legally delivered?

Try to post this query on the Ask the lawyer section....

Do they have process servers for hire - they would video and tape the contact - even if they refuse it they cannot say they didn't get it. If it is a big enough deal to be worth the cost anyway.

You do it by the sign on delivery method. The PO will return the signed receipt to you as legal proof of delivery. It doesn't matter who receives it as long as the delivery by the postman was at the address nominated and that the address has some connection with the intended recipient e.g. ID address, parents, place of business. The PO also scan the receipt so it is available on line - at least to lawyers.