The Courier IMAP Server

Copyright 1999-2008, 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.

• How the Courier IMAP server stores its mail, and works with your mail server
• The Courier IMAP server – Feature Summary

• Extremely small footprint. On x86, the Courier IMAP server’s main daemon is a fraction of the size of the UW-IMAP server, and has a greatly reduced memory footprint.
• Uses the Courier Authentication Library for authenticating mail accounts.
• Ability to restrict the maximum number of IMAP logins, and the maximum number of logins from the same IP address. This will prevent denial-of-service attacks where the attacker attempts to overload the server with multiple login connections.
• Shared folders. With additional server-side setup, folders can be shared between groups of accounts.
• IMAP over SSL. If OpenSSL or GnuTLS is installed, the Courier IMAP server will, optionally, accept IMAP connections over SSL. The IMAP STARTTLS extension is also implemented.
• Authentication using SSL certificates.
• IPv6 support, if available.
• IMAP extensions that implement server-side sorting. This can be optionally disabled in environments that have limited resources.
• The Courier IMAP server implements many popular IMAP protocol extensions that provide value-added functionality with most popular IMAP clients.
• Sorting and searching in non-English character sets. Download the source code to obtain a list of currently supported character sets.
• A compatible POP3 server is also included in the Courier IMAP server package, that uses the same authentication modules. The POP3 server can only access the INBOX, and cannot access any folders.
• An experimental mail access protocol, SMAP.
• IMAP/POP3 proxying. It is possible to distribute all mailboxes between multiple servers. A separate server (or a pool of servers) accepts all IMAP/POP3 connections, then connects to the right server based on the mailbox the connecting client is logging into.