Columns Style Main Window

  The Columns-style window is similar to the Outline style
  except that the window is divided into two panes.  The right
  pane can be scrolled vertically.  Each pane can be scrolled
  horizontally independently of the other.

  Click on the option (Outline, Paned, Tree, or Columns) to be
  used to display the index of messages in the main window.
  The default index style is Outline.

  To save the profile settings, click on the OK button, then
  choose Save Settings on the Options Menu.  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.

  Click on Cancel to leave the settings unchanged.

  Additional topics:

  *   Mail Window Options Dialog Box