Click on the toolbar.
(Optional) Size the QuickViewer by dragging a corner of the window or by dragging the horizontal dividing line up or down.
In the Mailbox, click each item that you want to read.
To close the QuickViewer, click on the toolbar.
If you do not want the bottom of your Folder List to be covered by the QuickViewer, click View > QuickView, then click Long Folder List.
For more information, see About QuickViewer.