speedbar is an Emacs-extension that shows navigational
information for an Emacs buffer in a separate frame. Using
mu4e-speedbar, mu4e lists your bookmarks and maildir
folders and allows for one-click access to them.
To enable this, add (require 'mu4e-speedbar) to your configuration;
then, all you need to do to activate it is M-x speedbar. Then,
when then switching to the 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.