Cyrus IMAP!

Dovecot is an open source IMAP and POP3 email server for Linux/UNIX-like systems, written with security primarily in mind. Dovecot is an excellent choice for both small and large installations. It's fast, simple to set up, requires no special administration and it uses very little memory.

Archiveopteryx is an Internet mail server, optimised to support long-term archival storage. It seeks to make it practical not only to manage large archives, but to use the information therein on a daily basis instead of relegating it to offline storage.

The Courier IMAP server is a fast, scalable, enterprise IMAP server that uses Maildirs. Many E-mail service providers use the Courier IMAP server to easy handle hundreds of thousands of mail accounts. With its built-in IMAP and POP3 aggregation proxy, the Courier IMAP server has practically infinite horizontal scalability. In a proxy configuration, a pool of Courier servers service initial IMAP and POP3 connections from clients. They wait to receive the client's log in request, look up the server that actually holds this mail account's mailbox, and establish a proxy connection to the server, all in a single, seamless process. Mail accounts can be moved between different servers, to achieve optimum resource usage. »

DBMail is a fast, scalable IMAP4 and POP3 server that stores all email and users in a Postgresql, MySQL or SQLite database.