The client is the same client that we're developing for the 5.0 release of Zimbra.
The server is essentially the 5.0 Zimbra server with a few important differences:
- It uses Apache Derby (a.k.a. Java DB) as an embedded SQL database rather than a separate standalone MySQL install
- It keeps a local cached copy of the user's account information in the Derby database instead of using a separate OpenLDAP server
- There's a builtin sync client that periodically pulls changes from the server and pushes local changes back up to the server