Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
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.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
Roundcube - Web-based IMAP email client
Mailpile - Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.
Microsoft Exchange Server - Microsoft Exchange Server is a messaging and collaborative software product developed by Microsoft.