Exchange Server 2007 introduces a new mechanism for distributing Offline Address Books (OAB) that does not require Public Folders. It instead uses HTTP(S) and the Background Intelligent Transfer Service (BITS) (For more information related to BITS, see About BITS). In an Exchange 2007 deployed environment, Outlook 2007 clients can download their OAB via a web distribution point published on a Client Access Server (CAS). Support for this new distribution mechanism is added to Hosted Messaging and Collaboration version 4.5, allowing organizations and users to be provisioned and configured to take advantage of the enhanced functionality.

There are several potential advantages of the new distribution mechanism including supporting more concurrent clients, reduced bandwidth usage, and more control over the distribution points.

Web-based OAB distribution has greatly improved scalability. It supports a magnitude increase of number of OABs per server, and allows Service Providers to increase operational efficiency by reducing both hardware, software licensing, and administration cost. At the service level, web-based distribution can be added as a new plan feature, and offered at a discounted price to incent customers to migrate to Outlook 2007, and help reduce legacy client support for Service Providers.

It is important to note that the new distribution mechanism requires Outlook 2007, but you can always choose to use both Public Folder Distribution and Web-based Distribution of OABs. That way, older clients can still access their OABs using Public Folders while Outlook 2007 clients can take advantage of the enhanced functionality.

At the provisioning level, MPS is updated with new providers and namespaces to support changes in technology. Fault tolerance protection against distribution point failures is added as the new benefits of the web-based distribution.