Replying with the Current Message

  Choose Reply Using This Message from the Read window's
  Create-Send menu to display the current message in a
  Create-Send window.  Mail automatically enters some of the
  address information in the Create-Send window.

  In the To entry box, Mail inserts the name of the sender of
  the original message.  In the Subject entry box Mail inserts by
  RE: followed by the subject of the original message.

  Edit the text of the original message, if desired, and enter
  the text of your message.  Click on the Send button when the
  message is complete.

  Additional topics:

  *   Replying to a Message