Copyright 1999-2015, Double Precision, Inc. This software is distributed under the terms of the GNU General Public License. See COPYING for additional information.
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.
The only practical limitation on the Courier IMAP server is available network and I/O bandwidth. If you are new to the Courier IMAP server, this may sound a bit intimidating. But you do not need to tackle everything at once. Start by taking small, easy steps. Your first step will be to set up a small the Courier IMAP server server, using it like any other traditional IMAP service, on a single server. After you gain experience and become comfortable with Courier, you can then begin exploring its advanced features.
This is the same IMAP server that's included in the Courier mail server, but configured as a standalone IMAP server that can be used with other mail servers - such as Qmail, Exim, or Postfix - that deliver to maildirs. If you already have Courier installed, you do not need to download this version. If you install this version, you must remove it if you later install the entire Courier server.
The following pages provide additional background and supporting documentation. You can jump anytime to the download page using the top menu bar.