Courier-IMAP is a fast, scalable, enterprise IMAP server that uses
Maildirs. Many E-mail service providers use Courier-IMAP to easy handle
hundreds of thousands of mail accounts. With its built-in IMAP and POP3
aggregation proxy, Courier-IMAP 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 Courier-IMAP is available network and I/O
bandwidth. If you are new to Courier-IMAP, 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
Courier-IMAP 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.