Sending SMS text messages also requires receiving Delivery Reports (DLR) from carriers or operators. Most people will have their phones turned on at some point during the day, so if your message goes a whole 24 hours without being delivered, then it's fair to assume that a switched-off phone is not the explanation. If this reoccurs, then it can be hard to know whether the recipient's phone is switched off, or there is a more serious problem with either your phone or theirs. At this point, you can try and resend the message, and the process will be repeated. If you do get a 'Failed' warning on one of your messages, then you know that the delivery has definitely not been successful.

If the message still won't go through, it is eventually marked as 'Failed.' For example, Messente's system is configured to try to deliver messages within 6 hours. If a certain amount of time passes without the SMS being successfully delivered, your mobile carrier might decide to stop trying. When the phone is available again, the message gets delivered. But when the SMS doesn’t get delivered on the first try, it is automatically re-sent in certain intervals. If you get a busy signal while calling, you try again later.

Messages not delivered mac glitch