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Zimbra Collaboration is an open, secure collaboration tool for email, calendaring, tasks and more. With the most innovative web application available today, Zimbra boosts the productivity of users on any desktop and dramatically reduces TCO compared to legacy platform vendors.

A fully fledged, free email server solution, an open source project (GPL v2).

Microsoft Exchange Server is a messaging and collaborative software product developed by Microsoft. It is part of the Microsoft Servers line of server products and is widely used by enterprises using Microsoft infrastructure solutions. Exchanges major features consist of electronic mail, calendaring, contacts and tasks; support for mobile and web-based access to information; and support for data storage.

Axigen is a premium, all-in-one email, calendaring and collaboration platform, based on an innovative, proprietary technology, providing unmatched manageability for system administrators. Integrating all modules in a robust messaging solution, Axigen is developed for demanding users, from small businesses to large service providers.

OpenChange aims to provide a portable Open Source implementation of Microsoft Exchange Server and Exchange protocols.

OBM is a free messaging and collaboration platform useful for just a couple to many thousands of users with strong support for mobile devices.

Turnkey Solutions for the Enterprise Cloud

Atmail allows you to choose between on-premises or cloud deployment. Features include email, contact, calendar, task, storage, synchronisation and provisioning.It's extensible through a RESTful API and allows easy rebranding and white-labelling.

Xeams is a secure & powerful mail server for Windows, Linux, Solaris, MacOSX and other flavors or UNIX, which supports SMTP, POP3, and IMAP. It includes a powerful spam/junk filtering engine that elimates up to 99% of junk email right out of the box. The software offers a flexible approach to email, making it a very user friendly server.