Cached Exchange Mode is a new feature in Outlook 2003.

When you turn on Cached Exchange Mode in Outlook 2003, and when the connection from your computer that is running Outlook 2003 to the Exchange Server 2003 computer is not available, Outlook switches to the Trying to connect state or to the Disconnected state. If the connection is restored, Outlook switches to the Connected state or to the Connected (Headers) state. Any changes that you made while you were offline are synchronized automatically when a connection to a server is available. You can continue to work while changes are synchronized.

Outlook 2003 Cached Exchange Mode offers you the following benefits:

• After messages have been cached locally, typical user operations do not cause interactions that block the server.

• Quickflagging, marking a message as read, replying, and editing require a small amount of data to be pushed up to the server to keep the mailboxes synchronized. However, the pushing of data occurs in the background. This behavior causes much faster access to messages and to attachments, because you work from the local copy instead of the server copy.
 
• Additionally, Cached Exchange Mode causes no loss of conventional functionality. New e-mail notifications, full Global Address List details, free/busy lookup, public folder access, and delegate support function as expected. However, this is true only when a network connection to an Exchange Server computer is present.
 
• Cached Exchange Mode provides intelligent use of bandwidth. This functionality is enabled by synchronizing only headers on slow connections (connections that are slower than 128 kilobits per second [Kbps]). This functionality works only when a network connection is present. 

Additionally, Cached Exchange Mode offers administrators the following benefits:

• Reduced server load. After messages are cached locally, re-opening the same message does not require server transactions.
 
• Reduced network load. After messages have been pulled over the network one time, subsequent access to those messages does not cause additional network traffic. Because messages are also compressed, there is an additional reduction on network load.