Ristretto is the mail API that is used by Columba, a Java mail client (http://columba.sf.net). It is available as a stand-alone API, and it provides a well-tested, slim, and fast Java implementation of all relevant mail protocols and features. It is licensed under a MPL/LGPL/GPL tri-license.
IMAPEngine is an IMAP server that uses a tdbengine database structure to store all accounts, messages, and mailboxes. This means it has nearly no limits in numbers, sizes, and performance, and it uses indexes to sort/find its data. IMAPEngine is IMAP4Rev1 compatible and also has built-in support for the ACL, ID, IDLE, and UIDPLUS extensions. It is not (yet) an MTA, but it retrieves incoming messages from different POP3 servers. An simple integrated Web interface makes account configuration easy.
Openmailadmin is a little administration interface to every complete IMAP mail server daemon. It supports every feature IMAP provides, and fits in most MTA configurations. A key feature is the non-standard, generic administration hierarchy which not only seperates "normal users" from "administrators", but enables the mailserver-master to create instances between them. You will be able to let other users create their own sub-users and thus either share a single mail server between different organizations or project your company's employee structure. It excels with features such as regex addresses and folder ACL management.
imap-purge is a little tool that automatically purges mailing list folders in an IMAP account. With very busy mailing lists such as the Linux kernel list, folders fill up quickly. The tool was written for the purpose of cleaning up individual folders that were restricted by a quota. You can specify a certain time frame (such as the last thirty days) or a maximum number of messages for each folder.
Teach-SA reads mail in designated maildir folders (spam on the one hand, ham on the other) and feeds them to Spamassassin for Bayesian learning and submission to various spam detection schemes. It is good for implementing any sort of supervised training in addition to Spamassassin's unsupervised training (also known as automatic whitelist) while reducing training-related admin workload to nearly zero. It fits any setup storing mail as maildir, but could trivially be modified to work with mbox based systems. It was originally developed so that users could report spam by moving it to a specific folder in their own IMAP mailbox.
Knight's Mail is a Web mail application that uses XUL to render the client display. Its goal is to look and feel as close to Mozilla Thunderbird as possible, while maintaining the positive qualities of a Web mail client such as keeping settings on the server and only requiring a (Mozilla-based) browser to be installed on the client.