Applying Window Option Settings

  Click on the Apply button to view your changes without
  closing the dialog box.  Changes to the profile settings are
  saved for this session only.  To save the profile settings
  permanently, choose Save Settings on the Options Menu.

  Changes to the Startup State option do not take effect until
  you exit and reenter Mail.  If you are changing the Main
  Window Style option to or from Paned style, the changes do
  not take effect until you exit and reenter Mail.

  Additional topics:

  *   Mail Window Options Dialog Box