As an alternative to the mouse you can use the following keys:
To switch between the menu bar, the folder tree, the sidebar, 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 sidebar, 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 toolbar, the sidebar or the display area, use the cursor keys.
To select a function, an input field or a checkbox, 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 popup or cancel a dialog window, press [Esc].
To delete the selected E-Mails, folders or files press [Del] or [Backspace].
To move selected E-Mails to the Archive folder, press [a].
To select all objects in the sidebar, select one object. Then press [Ctrl]+[a].