Zimbra is not so much an Exchange alternative replacement software, but its own email and collaboration suite of software which may even surpass the capability of Exchange.
Zimbra Collaboration provides business-class email, shared calendaring, file sharing, and tasks via a web based interface with full mobile device integration support. The solution is fully open source over the web interface but requires commercial licensing for email client integration and offline support.
An existing feature of Zimbra is its open API and use of Zimlets to extend capability. This allows it to both interface directly with third party CRM software such as Sales force and also social media and forums.
We will implement and support an on premise Zimbra installation or host a tailored private cloud solution on our own cloud platform.