Zimbra - Zimbra is trusted by over 500 million users to increase productivity with a complete set of collaboration tools while maintaining total control over security and privacy.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
mailcow - An open source mailserver suite.
Roundcube - Web-based IMAP email client
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
Mail-in-a-box - Mail-in-a-Box provides webmail and an IMAP/SMTP server for use with mobile devices and desktop mail software and also includes contacts and calendar synchronization.