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Change preferences in Messages on Mac
Use Messages Preferences to set up and turn on iMessage and change options for sending and receiving text messages.
In the Messages app on your Mac, choose Messages > Preferences, then click a preference pane:
General: Change general options such as the sound that plays when you receive a text message and the default text size.
iMessage: Set up and turn on iMessage. Specify options such as how you can be reached for text messages and if you would like to send read receipts.
To learn about the options in a pane, click the Help button 
at the bottom of the pane.