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).
Trojita is a Qt IMAP email client. It is a pure Qt4 application with no additional dependencies. It has a robust IMAP core implemented using Qt's Model-View framework. Standards compliance is a design goal. It has support for bandwidth-saving mode aimed at mobile users with expensive connections. In addition to the usual SSL/TLS connections, the server can be accessed via SSH. It has on-demand body part loading, offline IMAP support, and safe dealing with HTML mail (actually more robust than Thunderbird's).