Your delivery status comes directly from the networks as they get them from the users handset, here we've got an explanation for the most common delivery statuses:
Your message has successfully been delivered to the handset.
Unable to deliver your message (e.g. out of signal / phone switched off / inactive number).
Sent to Network
Your message has been sent to the mobile network, but the network has not successfully delivered the message as of yet. The networks will continually retry over a period of 48 hours to deliver the message.
The networks have been unable to deliver your message to the handset.