If you want to reply to a message sent to you, select the piece of mail in the InBox and then click on the “Reply” button.  Reply will automatically select the correct user from the user list and copy the mail’s subject line into the OutBox with a “Re:” inserted before it.  

Hold down the option key when clicking on the “Reply” button if you want the text of the mail that was sent to you to be included with your reply.  You may find it useful to double-click on the mail item before replying to it, allowing you to view the whole text of the mail as you compose your response.