speedbar is an
emacs-extension that shows navigational information
emacs buffer in a separate frame. Using
mu4e lists your bookmarks and maildir folders and allows for one-click access to
mu4e loads mu4e-speedbar automatically; all you need to do to activate it is M-x speedbar. Then, when then switching to the Main view, the speedbar-frame is updated with your bookmarks and maildirs. For speed reasons, the list of maildirs is determined when mu4e starts; if the list of maildirs changes while mu4e is running, you need to restart mu4e to have those changes reflected in the speedbar and in other places that use this list, such as auto-completion when jumping to a maildir.
mu4e-speedbar was contributed by Antono Vasiljev.