OBM - OBM is a free messaging and collaboration platform useful for just a couple to many thousands of users with strong support for mobile devices.
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.
Microsoft Exchange Server - Microsoft Exchange Server is a messaging and collaborative software product developed by Microsoft.
Microsoft Office 365 - Microsoft Office is an office suite of desktop applications, servers, and services for the Microsoft Windows and OS X operating systems.
CrossBox - Designed for those who settle only for the best — CrossBox is beautiful, dazzlingly fast, and comes with advanced features that provide you and your team with cutting-edge communication technologies.
Kolab Community - Kolab Groupware is a secure, scalable, reliable, flexible, full-featured collaboration suite.