AstraCom Solutions offers a unique alternative to Microsoft exchange in the form of the PostPath server.

The PostPath Server™ is a Linux-based enterprise-class email and collaboration server. It is the only drop-in and plug-compatible alternative to Microsoft Exchange™

Drop-In Compatibility

From a technical perspective, "drop-in and plug-compatible" means that the PostPath Server™ supports Exchange™-compatible network protocols at the server. The practical effect is that Microsoft or Microsoft-enabled software (whether Outlook™, Active Directory™, existing Microsoft Exchange™ servers, or other applications like Blackberry Enterprise Server™) interoperate with the PostPath Server just as they would with Microsoft Exchange™.
This allows a PostPath Server to be "dropped in" to an existing infrastructure without having to change anything already there. The existing Active Directory™ tools can be used to migrate users from Exchange to PostPath; there is no need to touch the desktop, and of course no client-side plug-ins or connectors while users enjoy the full rich feature set of Outlook™, seamless interoperability with users on other servers (whether PostPath or Microsoft), and continuing access to their applications and services.