Postmaster is an Internet mail transport agent (MTA) that handles incoming ESMTP network connections and delivers accepted messages to the user's mailbox by piping it into an arbitrary local mailer (such as Procmail). It is configurable beyond anything you'll ever need, handles any number of concurrent connections without forking or using OS threading, and it is pretty efficient. It currently lacks any form of queue management, but for leaf sites that don't need to do extensive mail relaying, it is a reliable and powerful solution.
Elmtree Express Server is a realistic alternative to the Microsoft Windows server plaftorm. Express Server is a workgroup server solution, providing your business with integrated email, print management, and data storage functionality, all in one easy package. It includes an email server, a central address book via LDAP server, a DHCP server, a central file server, a print server, automated daily backups, GUI administration tools, and more.
Exim-Python is an extension to Exim that adds the ability to execute compiled Python functions and methods from within Exim configuration file string expansions. Since Exim uses string expansions heavily throughout it's operation, this feature allows powerful added control to an already very flexible mail server.
CAKE (Key Addressed Crypto Encapsulation) is a bunch of Python and C++ for implementing the CAKE protocol. The CAKE protocol is a protocol in which all messages have source and destination addresses that are public keys. This is in the same way that IP packets have a source and destination address that is an IP address.