The '[+]' link turns on the folder tree on the mailbox screen.

Pressing the '[-]' will hide the folder tree from the mailbox screen.

You can show/unhide the folder tree at any time.

The dropdown menu on the Mailbox Window is only for one-time changes. In order to change it permanently go to Options and then the Folders screen. There you can set the 'Rows to display' for each folder. Columns in folders cannot be set to any sort preference permanently.

To turn off message preview, go to the Options screen and select Folders. You can have no preview, a 1-line preview, or a 3-line preview for each mailbox.