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Dovecot is an open source IMAP and POP3 email server for Linux/UNIX-like systems, written with security primarily in mind. Dovecot is an excellent choice for both small and large installations. It's fast, simple to set up, requires no special administration and it uses very little memory.

The Cyrus IMAP server differs from other IMAP server implementations in that it is generally intended to be run on sealed servers, where normal users cannot log in. Users can access mail through the IMAP/IMAP-S, POP3/POP3-S or KPOP protocols. The private mailbox database design gives the server considerable advantages in efficiency, scalability, and administratability.

hMailServer is a free e-mail server for Microsoft Windows. Its used by Internet service providers, companies, governments, schools and enthusiasts in all parts of the world.

The Mercury Mail Transport System is a mail server that sends and receives messages on behalf of a group of users on a machine or local area network. Messages from the outside world are received by Mercury and delivered to the users, who can access it at any later point. Messages sent by local users to the outside world are passed to Mercury, which then takes whatever steps are necessary to deliver them, removing the burden from the users workstation and allowing them to continue with other work. »

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. »

DBMail is a fast, scalable IMAP4 and POP3 server that stores all email and users in a Postgresql, MySQL or SQLite database.