Replying to a Message
  Choose Reply from the main window's Create-Send menu to
  reply to a selected message.
  To reply to a selected message:
  1.  Choose Reply from the main window's Create-Send
      menu.
      Mail opens a Create-Send window and automatically
      enters the address information for this reply.
  2.  Enter the text of your reply.
  3.  Click on the Send button to send the reply.