messaging
When I sent a message and I get a not delivered message back what does that really mean
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It really means the message was not delivered. This could happen for a number of reasons. Some include - no Wi-Fi or cellular connection, wrong Send To address/phone number, you are sending a iMessage to an Android phone and more. This link offers advice:
If you can't send or receive messages on your iPhone or iPad - Apple Support
I’ve copied the pertinent part here:
If you see a red exclamation point
If you try to send a message and you see 
with an alert that says Not Delivered, follow these steps:
- Check your network connection.
- Tap , then tap Try Again.
- If you still can’t send the message, tap , then tap Send as Text Message. Messaging rates may apply. Contact your carrier for more information.
You can set up Messages so that it automatically tries to send messages as an SMS when iMessage isn't available. Go to Settings > Messages, and turn on Send as SMS.