I have an iPhone 5 running iOS7. A friend has a new iPhone 5S also running iOS7. I am on AT&T and he is on Sprint (I think). Anyway, when I send him a text (via iMessage not SMS) he does receive my text, but I get a "Not Delivered" message in red. Does anyone have any ideas why this would be happening? He is the only one this happens with. Like I said, he DOES receive my texts, even though I'm told it was not delivered.