[cglong]

One underrated feature of iMessage is graceful degradation. If I'm unable to send/receive data for whatever reason, my message (and their replies) automatically fall back to SMS. There's been a few occasions where this was my only way of communicating important information, so I'm grateful for it.

[anoncake]

Does this also apply when the recipient cannot receive data?

[OkGoDoIt]

Yes. If the message is unable to be delivered to the recipient via iMessage, then it is sent as SMS instead. So for example if you try to send a message to someone and their phone is turned off or doesn’t have signal, it will eventually fail and then send as SMS.

This is all controlled via a setting in iOS iMessage settings “Send as SMS”, which I believe is turned on by default but I’m not sure. So your mileage may vary.