Using Multiple and Default Read Windows

  You can open more than one Read window at one time.  This
  lets you read multiple messages at once or read new mail
  without disrupting the message you are currently reading.

  The first Read window you open is your default Read window.

  It is identified by the symbol [D] in the window's title bar.
  Additional Read windows are identified by the number in
  their title bar; for example, Read 2, Read 3, and so on.

  The default Read window is used to display messages unless
  you specify another window.  For example, clicking on the
  Read New Mail button or double clicking on a message in
  the main window displays the message in your default Read
  window.

  New mail cannot be added to the INBOX if you are reading
  a message in the INBOX in a Read window other than the
  default Read window or if you are replying to a message in
  the INBOX.

  To read an old message in another Read window, select the
  message and choose In New Window from the Read menu.

  Additional topics:

  *   Using a Read Window