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.
Roundcube - Web-based IMAP email client
mailcow - An open source mailserver suite.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Mailpile - Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.
Modoboa - Modoboa is a mail hosting and management platform including a modern and simplified Web User Interface.