Under the hood, mu4e is fully search-based, similar to programs like
notmuch and
https://sup-heliotrope.github.io/sup.
However, mu4e’s user-interface is quite different. mu4e’s mail
handling (deleting, moving, etc.) is inspired by
http://www.gohome.org/wl/Wanderlust (another Emacs-based e-mail client),
http://www.mutt.org/mutt and the dired file-manager for emacs.
mu4e keeps all the ‘state’ in your maildirs, so you can easily switch
between clients, synchronize over IMAP, backup with rsync and so on.
The Xapian-database that mu maintains is merely a cache; if you
delete it, you won’t lose any information.