As an alternative to the mouse you can use the following keys:
To switch between the menu bar, the folder tree, the side bar, and the display area, use [Ctrl]+[F6] on Windows and Linux systems, [F6] on MacOS systems.
To move the cursor within input fields, use the following keys: cursor keys, [Home], [End].
To browse the folder tree or the side bar, use the following keys: [Page up], [page down]
To open or close a folder in the folder tree, use the left or right arrow key.
To select elements in the folder tree, the tool bar, the side bar or the display area, use the cursor keys.
To select a function, an input field or a check box, use the [Tab] key. The selected element will be highlighted.
With [Shift]+[Tab] you can select elements in reverse order.
To execute the function selected, press [Enter].
To enable or disable a selected checkbox, press the [Space bar].
To toggle the checkboxes, use the cursor keys.
To close a pop-up or cancel a dialog window, press [Esc].
To delete the selected E-Mail messages, folders or files press [Del] or [Backspace].
To move selected E-Mail messages to the Archive folder, press [a].
To select all objects in the side bar, select one object. Then press [Ctrl]+[a].