What does "when iMessage is unavailable" mean?

Solution 1:

It means either

• when the device is unable to determine if a message has been delivered over the iMessage transport successfully. When a successful delivery occurs you see a little 'Delivered' notification below the message in the history, and after a period you should have the option to resend as an SMS

• or the recipient has no registration with the iMessage server to indicate that it could use the iMessage transport and will use SMS instead.

Solution 2:

Another scenario I've discovered is when I'm connected to a WiFi signal that requires HTTP authentication. If I bring up the browser and authenticate, I'm fine. However, if I just let the iPhone connect to the WiFi, but I don't bring up the browser to authenticate, then the data network will be seen as dead, and any iMessage attempts will fallback to SMS (green dialog boxes). But then, as soon as I bring up the browser to authenticate, my conversation will switch back to iMessage (blue dialog boxes). So unless I remember to keep re-authenticating to the wireless network with the browser, my best bet it to just "Forget this network" and allow the iMessage to be sent over the AT&T data network, then it will remain as an iMessage.