exJello is a JavaMail provider that connects to a Microsoft Exchange server (actually, it uses the WebDAV interface exposed by Outlook Web Access). It is designed as a drop-in replacement for the standard POP3 and SMTP providers. This allows you to send and receive messages through your Exchange server in situations where a POP3/SMTP interface is not available (through a restrictive firewall, for example, or if your administrator simply does not provide a POP3 or SMTP gateway).
mailxel is a personal mail store and mail client with a focus on the management and organization of high volume mail boxes. The main features are fast search, tagging, GTD inbox management workflow, efficient data storage, address management, automated server-side mail folder organization, and normal mail functionality (send, receive, forward, etc.).
Lamson is an SMTP server adopts a modern Web application framework design. Rather than bizarre flat file "databases" and hashtable stores, Lamson uses anything that Python does to store data, including Django's ORM, web.py's simple database library, Tokyo Tyrant, Lightcloud, Python's Shelve (not recommended), Raw SQLite3, SQLObject, CouchDB, Mongo DB, and SQLAlchemy.
JWebEngine is an embeddable component that is far more advanced than the internal Sun HTML renderer since it flawlessly passes the Mail Accid and the Accid 1 test. It can easily be used as a JavaHelp or Mail Client renderer in your own applications. Even headless rendering is possible, for thumbnail generation for example. JWebEngine is already being used in the products i-net Crystal-Clear and i-net HelpDesk.
Bulk Mac Mail is an an application for sending personalized mass email messages. It allows you to use message templates with multiple custom fields from the recipient database inside your messages. Customer lists can be imported from files of any type. Since all messages go through the standard Apple Mail's message delivery framework, they are absolutely indistinguishable from the messages that were actually sent by Apple Mail.
Mireka is a mail server with SMTP, Mail Submission, and POP3 services. It can also be used as an SMTP proxy. As a proxy, it can help to prevent or diagnose mail problems, like outgoing backscatter spam. It provides detailed logging, basic mail traffic statistics, tarpit to prevent email harvesting, and loop detection. It can filter email by DNSBL, SPF, mail size, local domains, and recipients list. Local recipients can be specified using regular expressions. TLS is supported for incoming connections. Aliases, forward lists, and very simple mailing lists can be configured in XML. Custom filters and other components can be written in Java, and they can be easily implemented and installed.
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).
smtp-cli is an SMTP command line client with support for advanced features such as STARTTLS, SMTP-AUTH, or IPv6. It also has scriptable message composition capabilities supporting anything from simple plain-text messages right up to building complex HTML emails with alternative plain-text part, attachments, and inline images. It's also a convenient tool for testing and debugging SMTP servers' setups. smtp-cli can be used to check a given server's capabilities, to test your server's setup, or to create and dispatch email messages. The command line interface is intuitive, and everything is scriptable and can run in a completely non-interactive mode from scripts or cron jobs. smtp-cli is ideal for shipping log files from remote machines, running periodic mail delivery test loops, etc.