j-XChange is a pure Java implementation of the entire Collaboration Data Objects (CDO 1.21) library for accessing Microsoft Exchange Server in a platform independent manner. This library will allow interoperability with MS Exchange from all non-Windows platforms that support Java. This library is powered by j-Interop.
libSmtpThread is a multi-threaded C++ library for sending mail via SMTP. The library provides a connection pool that initializes a set of threaded SMTP connections. Emails can be pushed into the pool, which accepts all mail and delivers them to a free connection. The delivery to the SMTP server is processed in a separate thread, so you don't have to wait for mail to be accepted. It has been successfully tested with Postfix.
NmnNewsletter is a Web application designed to provide a simple but full featured newsletter and messaging manager for Web sites running on PHP 4.x or 5.x and MySQL. It implements lots of useful features including newsletter saving and editing, previewing and presending to test addresses, logs, and statistical data concerning newsletters and users alike. It also has some extra features like HTML to text auto conversion for sending multi-part messages, auto generation and insertion of an unsubscribe link, tracking of who reads your messages and when, and more.