When you double-click on a message line, the system brings up a window showing the contents of the message. If you want to send a response to a message you have received, there are two basic ways to do this. Select Reply from the Message menu. Write your response and send the message as usual. Reply will send your reply to all the original recipients of the message.
Embarrassing things can happen.
Select Reply with Quote from the Reply Special submenu of the Message menu. You can edit the quote, add your response and then send the message just as if you had written the whole thing. Quoting back all or part of a message is a courteous way to remind the other person what they said and why you are responding. Only that section will be quoted in the new message. Before you choose Send , be sure you want all of them to read your message.