The preview pane, the lower portion of the Mail viewer window, allows you to view a selected email without opening it in a separate window.
The preview pane is visible by default, but you may hide it by double-clicking the separator. You can adjust the size of the preview pane by dragging the separator up or down.
If the preview pane is hidden, or if its size is reduced, it may not be visible.
When shown, the mailboxes drawer appears on the left or right side of the Mail viewer window. You can show or hide it by clicking the Mailboxes button in the Mail viewer window. You can also hide the mailboxes drawer by dragging its outer border toward the Mail viewer window.
If the Mail viewer window is maximized to fill your entire display, it will not resize when you open the mailboxes drawer. Reduce the size of the Mail viewer window by clicking the green zoom button or by dragging the resize control in the lower right corner.
Want to move the mailboxes drawer to the other side of the Mail window? Simply click the Mailboxes button to hide the drawer, then drag the Mail window to the left side of the screen (if you want the drawer to "come out" on the right side) or the right side of the screen (if you want the drawer to "come out" on the left side). Click the Mailboxes button again to show the drawer.