dovecot

Dovecot is an open source IMAP and POP3 server for Linux/UNIX-like systems, written with security primarily in mind. Although it's written in C, it uses several coding techniques to avoid most of the common pitfalls. Dovecot can work with standard mbox and Maildir formats and it's fully compatible with UW-IMAP and Courier IMAP servers' implementation of them, as well as mail clients accessing the mailboxes directly. It's easy to migrate from them to Dovecot. Dovecot will also soon have its own high performance mailbox format called dbox. Perhaps some day in future Dovecot will also support storing mails in SQL databases.