When a message appears in your Outbox, it is proof that you have generated a message and successfully addressed and sent it to the recipient. When it shows in your Sent box, it is proof positive that it has been Opened by the recipient. That's how you know if you've been successful, or not, in sending a message.
Keep in mind, a recipient may or may not respond to your message, and may or may not read it if it is opened. The only way you can be assured that you are communicating with an individual is if you receive a response.