Adjusting Pane Size

  You can adjust the relative size of the panes.  Making one
  pane smaller makes the other pane larger.  This allows you to
  view more text in either pane without resizing the window.

  To adjust the panes, press MB1 on the pane knob (box) on the
  dividing line between the panes, drag the line to the desired
  location, and release MB1.

  Additional topics:

  *   Overview of Mail