Dovecot is an IMAP server whose major goals are security and extreme reliability. It uses index files to optimally store the mailbox state, which makes it very fast even with huge mailboxes. Indexes won't prevent external mailbox updates, so Dovecot is still fully compatible with standard Maildir and mbox formats. There's also a fully featured POP3 server included.
Kerio MailServer is a secure corporate Internet messaging server for SME. It integrates SpamAssassin, McAfee anti-virus, SSL encrypted POP3, IMAP4, SMTP, WebMail (HTTP) and WAP protocols, attachment/content filtering and a MAPS/user-defined anti-spam database. It supports multiple domains, automated replies, SMS notifications, shared and public folders, email folder backup, full Active Directory implementation, mailing lists, and shared contacts. It runs on Red Hat 7 and 8, Windows 98/ME/NT/2000/XP, and Mac OS X 10.2, and is available with English, French, Dutch, and German user interfaces.
SmartSieve is a Web-based graphical user interface for creating and managing sieve scripts on a Cyrus IMAP mail server. It provides a way for non-technical users to generate sieve rules for mail filtering and vacation messaging without prior knowledge of the sieve scripting language. It is written in PHP and is intended to be intuitive to use and simple to configure, and is compatible with Websieve scripts.
MIR (mobile IMAP reader) is a WAP frontend for reading email from various IMAP accounts. You can browse and read email messages stored in different IMAP mail folders. It is developed with efficiency in mind, and optimised to present all information on a mobile phone in a compact form without any overhead.