Subfolders & Folders Tree Directory/Dropdown Style List View Option

K9 desperately needs a settings display option for folders and subfolders to display as tree directories/dropdown style lists instead of just one gigantic single-line list. Some of us have and use MANY folders and subfolders!!! The gigantic single-line list makes it extremely difficult to navigate around our folders and subfolders. Thunderbird has it set up this way, why not K9? Thanks!