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
Microsoft Exchange Server - Microsoft Exchange Server is a messaging and collaborative software product developed by Microsoft.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Blue-Mind - Blue Mind is a complete solution of company messaging, calendars and collaborative work.
Rainloop - RainLoop is a web based email client.