Microsoft Exchange Server Alternatives

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.

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

Web-based IMAP email client

hMailServer is a free e-mail server for Microsoft Windows.

9 hours ago - It does include a proprietary software set optimized for business users, students, researchers and developers." A list of features and further details can be found in the company's release announcement.

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An open source mailserver suite.

Fuduntu is an open-source appropriation of Linux, a remix of the Fedora working framework, which thusly depends on the honor winning Red Hat Enterprise Linux OS.

Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.

CrunchBang Linux is a Linux system that got from Debian.

OpenMandriva Lx is the most recent version of OpenMandriva, a desktop Linux appropriation got from Mandriva Linux.

Haiku is an open source OS catered specifically to the needs of personal computing.

Kolab Groupware is a secure, scalable, reliable, flexible, full-featured collaboration suite.