This week, Microsoft will release a beta version of Exchange Server 5.5 that users can download from the Web. The new version, dubbed "Exchange Server 5.5 Release Candidate 1," includes an unlimited message store, and an improved backup. Previously, the message store was limited to 16GB. When Exchange Server 5.5 ships later this year, it will include support for Microsoft Cluster Server (found now in NT Enterprise Edition, and later in NT 5.0). It will ship with Outlook clients for Windows and the Macintosh, as well as with a Web-based Outlook Web Access client