I was recently trying to send an instant message to a colleague via the Messages app on my iPhone. She wasn't responding, and then I noticed that I got an error message indicating that it wasn't delivered. What was up?

Turns out Messages makes decisions about how to send your message based on what it knows about the sender and recipient's connections. Sometimes it will send SMS text messages but it will usually try to route them over the Internet through their own servers from one user to the other. It can be confusing since the same app is used for both message types and Apple doesn't specifically tell you which method it chooses. It does tell you though.

Notice the message with the error above: it has a blue background. That means Messages is sending it out over the Internet. The message to the right has a green background. That means it's sent with SMS, directly through the phone system.

As a general matter, it's a good idea for Messages to try to use the Internet where possible. The cost of messages over the Internet (or at least the marginal cost) is zero. SMS Text messages use SS7 signaling over the phone network and carriers charge for them. It may be that you have an unlimited texting plan so your marginal cost is zero, but you did pay for the plan.

But the opportunities for things to go wrong with messages sent over the Internet are many, and so such messaging is less reliable.

Would you like to change Messages so that it only uses SMS messaging? Here's how:

Now Messages will only use SMS.

Thanks to Neil McGrath of Acision for pointing this out to us.