conv2047 is a very simple RFC 2047 Unicode email header field converter. It reads header lines from standard input and tries to decode or encode them. Operation modes: -d decode (MIME-Header --> UTF-8) and -e encode (UTF-8 --> MIME-Header). The -c option can concatenate multiple output lines into 1.
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).
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.