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.