When Apple introduced the iPhone in 2007, iMessages was unveiled as a new way to send texts. In the Messages app, iMessages are displayed in blue bubbles, as opposed to the green bubbles of traditional SMS messages. Sometimes Messages will initially show the outgoing text in a blue bubble and then change it to a green one, indicating the message was sent via SMS.
Because of its advantages, iOS defaults to sending iMessages unless the service is turned off. When you select a recipient, your iPhone will check to see if the recipient's number or email is registered with the iMessage service. If it is, your text will automatically be sent as an iMessage. If you leave iMessage turned on, you should also enable the option to Send As SMS if iMessage is unavailable.
If you have a contact who previously had an iPhone but has since switched to a different device, their number or email may still be registered with the iMessage service. When you text them, your iPhone will still attempt to send iMessages, reverting to SMS when the iMessage fails to go through.
Because iMessages rely on data, there may be times when you have data or Wi-Fi access but no cellular service. When this occurs, your iPhone may attempt to send iMessages regardless of the recipient's phone.
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