By default, Outlook installs in cached mode. In cached mode, a copy of your mailbox is stored on your computer. This copy provides quick access to your data and is frequently updated with the mail server.
Cached Exchange Mode allows you to work with your Exchange account data when your connection to the Exchange server is interrupted or unavailable. Cached Exchange Mode stores a copy of your mailbox on your computer by automatically creating and using an offline folder file (an .ost file), into which it downloads and maintains a synchronized copy of the items in all the folders of your mailbox. Outlook automatically manages your server connection and data updates; when your connection is restored, Outlook synchronizes your cached mailbox with your mailbox on the server.
However, you still have the option to manually set the connection behavior in order to control the amount of data transmitted to or from the Exchange server. For example, if you use a service that charges you by the amount of data you send and receive (common with cellular and GPRS connections), you can select the Download Headers option to minimize the amount of data sent over the connection, and reduce your connection time. From the headers, you can choose the full items you need.
NOTE: If you use third-party applications that interface with your Outlook profile (e.g., PDA synchronization software, spam filtering software, Outlook add-ins), UITS recommends that you research these applications to make sure they are compatible with Cached Exchange Mode before enabling it.
However, having the local “cached” copy of your email presents an issue that some people do not like. That is, mail that is in the local cached copy can be read before (or without) logging into the Exchange server. New emails cannot be accessed, and you cannot send emails from the account until the user logs into the Exchange server, but the previously cached mail messages could be read by somebody else if you left your computer unlocked and unattended. This is no different than somebody reading your personal documents on your computer if you left it unlocked and unattended. It is recommended that you always log off or lock your computer when you will be away from it. It is also recommended that you leave cache mode enabled because Outlook will be much more responsive while you are using it. However, if the issue mentioned above is of concern to you, disabling cache mode is a simple process.
For more information see detailed instructions with screen shots.
In 2007, from the Tools menu, select Account Settings.
Highlight your Exchange server account, and then click Change
To turn Cached Exchange Mode on or off, under "Microsoft Exchange Server:", check or uncheck Cached Exchange Mode. Click Next, and in the window that opens, click OK
Click Finish. Restart Outlook for the change to take effect.