Replying to the Current Message

  Clicking on the Reply button in a Read window lets you reply
  to the currently displayed message.

  Mail opens a Create-Send window.  Mail automatically
  enters information in the To and Subject entry boxes, based
  on information in the original message.

  By default, the original message is not included in the reply.
  To include the original message in the reply, choose Include

  Original from the File menu in the Create-Send window.

  Enter the text of your reply.  Click on the Send button when
  the message is complete.

  Additional topics:

  *   Using a Create-Send Window