Q: I have an iPhone, I sent an iMessage to another iPhone. If the other iPhone is off will an iMessage be sent or will it be a text message in a green bubble?
A: The Messages app on an iPhone will always try to send an iMessage first, as long as the number you are sending to has been registered for iMessage. If the iMessage cannot be successfully delivered within a few minutes, it will fall back to sending a standard text message (in a green bubble) by default.
Unlike iMessages, since SMS messages do not rely on a data connection, this allows messages to be sent even when an iPhone may be out of data coverage, or have cellular data disabled. Of course, if the iPhone is completely OFF, then the message won’t be delivered until it is turned back on anyway, but iMessage doesn’t know the difference between an iPhone that’s turned off and an iPhone that’s simply wandered out of a data coverage area; the Messages app simply realizes that it can’t deliver the iMessage for whatever reason and decides to use SMS as a secondary method to get the message out as quickly as possible.
You can disable this “fall back” however by going into the Messages section in your iPhone Settings app and disabling the “Send As SMS” feature. This will result in an iMessage not being delivered until the destination iPhone is back in data coverage.
It’s also possible to force your iPhone to send an out as a text message instead, as long as it hasn’t already been delivered via iMessage. To do this, simply tap and hold on the undelivered message and a “Send as Text Message” option should appear in the context menu. This works even when “Send as SMS” is disabled in your settings, allowing you to decide when you’d rather send a text message for expediency or simply leave it to wait until the recipient’s device is back online.
Note that if the person you’re sending a message to has other devices set up to use iMessage with the same Apple ID, such as an iPad or Mac, your message will get delivered via iMessage to those devices if they’re online, even though the recipient’s iPhone may be off or out of data coverage. In this case, the option to send as an SMS instead will not be available since as far as your iPhone is concerned the message has already been successfully delivered via iMessage. The message will still show up on the person’s iPhone once they return to data coverage, however. See iMessages showing as Delivered when iPhone is out of coverage for more information on how this works out.