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Courier
Courier evolved out of several related projects, that merged together. Courier implements SMTP extensions for mailing list management and spam filtering. Courier can function as an intermediate mail relay, relaying mail between an internal LAN and the Internet, or perform final delivery to mailboxes.
Courier uses maildirs as its native mail storage format, but it can also deliver mail to legacy mailbox files as well. Courier's configuration is set by plain text files and Perl scripts. Most of Courier's configuration can now be adjusted from a web browser, using Courier's web-based administration module.
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. The only practical limitation on Courier-IMAP is available network and I/O bandwidth.