Could I use messages on MacOS 10.13.6?
I need to send text messages to my sister who only uses text messages.
Earlier Mac models
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There is a Messages application on High Sierra. It can handle both iMessages and SMS text messages.
iMessages travel between Apple devices, over the Internet.
SMS messages are a generic cell phone thing that will work with iPhones, Android phones, and even "dumb" cell phones. They travel over the cellular phone network. A Mac isn't a cell phone, so to send or receive this type of message, your Mac will need the help of an iPhone that is signed into iMessage using the same Apple ID.
Set up iPhone to get SMS texts on Mac - Apple Support
What is the difference between iMessage and SMS/MMS? - Apple Support