Outlook Cached Mode (2003 and later) improves performance
Cached mode is available in Outlook 2003 or later. It allows offline usage and usually improves Outlook mailbox performance.
Optimizing Outlook 2007 Cache Mode Performance
For very large mailboxes (greater than 2 GB), Outlook may slow down due to the size of the file used to cache your mailbox on your computer. This blog post explains one technique to optimize your Outlook 2007 cache mode performance.
Enabling Cached Mode in Outlook 2003
Cached Exchange Mode, available in Outlook 2003, is similar to offline folders in previous versions of Outlook, though easier to use. (For more information about offline folders, see the Knowledge Base document In Microsoft Outlook for Windows, what are offline folders, and how do I enable and disable them?) 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. It does this by automatically creating and using an offline folder file (a .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.
To enable or disable Cached Exchange Mode, follow these directions:
In Outlook, from the Tools menu, select Email Accounts… .
In the window that opens, under “Email”, make sure the radio button next to View or change existing email accounts is selected, then click Next.
Highlight your Exchange server account, then click Change… .
To turn on Cached Exchange Mode, under “Microsoft Exchange Server:”, check the box next to Cached Exchange Mode. To turn it off, remove the checkmark from the box. Click Next, and in the window that opens, click OK.
Click Finish. You must restart Outlook for the change to take effect.