What does “Sent as SMS” mean?

“Sent as SMS” means your iPhone could not deliver the message through iMessage, so it sent it as an ordinary text message instead. The message went through. Nothing is broken, and the recipient received it.

Why it happens

iMessage travels over the internet between Apple devices. SMS travels over the cellular network to any phone at all. When the first route is unavailable, your phone quietly takes the second one — and tells you it did.

• No data connection. iMessage needs Wi-Fi or mobile data. Underground, on a plane, or in a dead spot, there is nothing for it to travel on.
• The recipient is not on an Apple device. An Android phone cannot receive iMessage at all, so every message to it goes as SMS or MMS. These threads are green from the start.
• iMessage is switched off — on your device or theirs.
• The send timed out. iMessage tries first; if the attempt does not complete quickly enough it falls back rather than leaving the message stuck.
• The recipient's device is signed out of iMessage, for example after a factory reset or a switch to a non-Apple phone.

What actually changes when it falls back

The one that catches people out is cost. Domestic texts are included in most modern plans, but international messages and messages sent while roaming are frequently charged separately — so a long thread that silently fell back while abroad can show up on a bill.

How to stop it (and why you probably shouldn't)

iOS has a setting called Send as Text Message, in the Messages section of Settings. It is on by default. Switching it off stops the fallback: messages that iMessage cannot deliver will fail and show “Not Delivered” instead of going out as a text.

That is rarely what you want. The fallback exists so a message reaches the person when the preferred route is unavailable — turning it off trades “it arrived, slightly plainer” for “it did not arrive”. The one case where switching it off makes sense is deliberate cost control while travelling, and even then, turning it back on afterwards is easy to forget.

For the underlying mechanics of the fallback route, what is SMS walks the path a text actually travels, and what does SMS mean has the short definition and the comparison table.