iMessage fails to send despite 'delivered' iMessages that claim to have been 'delivered' then fail after going into airplane mode ~30 seconds later. What does 'delivered' actually mean, if not that the message was delivered to the recipient successfully? Why does my phone's connection still need to be active after that point? 

Observed behavior:


Send iMessage.
'Delivered' tag appears on the blue message almost immediately.
Put phone into airplane mode ~30 seconds later.
A few minutes later, the message shows up as a green text message instead, with a 'failed to send' error. Hitting 'try again' sends the message as a text.


Expected behavior: Step 4 above does not occur.
Blue print applies to another iPhone. Green print is a non-iPhone. The users phone connection could be down for some reason or out of range of a cell tower. You get a Delivered message but later get a message that the sent message could not be delivered. If a blue message arrives immediately is could be you've lost connection or phone is hidden from cell tower. Check your signal strength and try again.