As per the documentation for the sending service says: if you receive “Success:true” the call worked.
Then, you must check for “ExitErrorLevel” which, if it’s a success, reached a node where the service is confident that what needed to be done, got done.
If there is an error in your DATA or the MESSAGE, as can happen, the SERVICE will not tell you, because it does not know.
Its job is to save the data in the proper tables and issue the order to send the message.
The message is sent asynchronously.
If it fails for various reasons, you will see such reason in the contact’s history page, or in the next day's SendLog extractions.