mu is a collection of utilities for maildirs, in particular for indexing and searching them. mu fills a database with information about your emails, which can then be queried using an easy query language. It can work stand-alone or be integrated with a mail client. It also provides an emacs-based email client ("mu4e") and bindings for Guile/Scheme.
VM is a mail reader written in Emacs LISP for GNU Emacs and XEmacs. It can retrieve mail from local spool files or remotely via POP and IMAP, display and send MIME messages, thread messages, auto-sort messages into folders, and manipulate "virtual folders" of messages matching certain criteria. In Emacs versions with X support it provides an interface with toolbars, menus, and mouse-based message selection.