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.
qmailcheck looks for new mail in multiple IMAP mail boxes. If you get a lot of mail and split the messages into separate boxes (typically on the server side using procmail or sieve), then checking all the boxes by hand can be tedious. QMailcheck displays the results in a formatted list, highlighting the newest items. When new mail arrives in a folder, it is capable of raising the window, issuing a system notification, flashing the LED (on phones only), and vibrating (on phones only), all on a per-folder basis.
libquickmail is a library intended to give C/C++ developers a quick and easy way to send email from their applications. It supports multiple To/Cc/Bcc recipients and multiple MIME attachments without size limitation. It uses libcurl for SMTP communication, but a light version is also included without the libcurl dependancy. The library can also be used to just create a multipart MIME message body. It also comes with a command line application for sending mail (with optional attachments).