Microsoft KB Archive/252430

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Knowledge Base

OL2000: (CW) What Is an OST and Why Does It Increase in Size?

Article ID: 252430

Article Last Modified on 10/8/2003


  • Microsoft Outlook 2000 Standard Edition

This article was previously published under Q252430

NOTE: The procedures in this article only apply if you have installed Outlook with the Corporate or Workgroup option. With this option, you can use Messaging Application Programming Interface (MAPI) services. To determine your installation type, click About Microsoft Outlook on the Help menu. If you have the Corporate or Workgroup installation, you see "Corporate or Workgroup".


This article discusses offline folder files (.ost) and why they grow in size. The default location for your .ost file is your Windows folder.


When you use Outlook with the Microsoft Exchange Server information service, you have the ability to work with folders offline; you can use the contents of a folder without a network connection. You also have the ability to synchronize your offline folders and your primary message store when you are connected to a network.

When you synchronize the .ost file and your primary store they contain the same information. This allows you to work offline when the server is unavailable. When you reconnect to the server, changes made offline will be uploaded to the server, and mail received while offline will be downloaded to the .ost file. Only one .ost file can be used at a time.

The size of the .ost file is equal to the size of the folders you select to synchronize. By default, the .ost file will contain a copy of your Exchange mailbox (Calendar, Contacts, Deleted Items, Inbox, Tasks, and so on). As your mailbox grows in size, your .ost file also grows. The .ost file can contain other server-based folders such as the public folders. Each of these folders contributes to the size of the .ost file. The more folders you synchronize, the larger your .ost file will be.

The .ost file can contain as many root folders as you want, and each folder can contain a maximum of 64 kilobytes (KB) of subfolders. The .ost file has a 64-KB large table folder structure by default, as this is the Microsoft Exchange Server mailbox settings. The total file size of an .ost file cannot exceed 2 gigabytes(GB).

Like personal folders, the .ost file can be compacted to save space on your computer. The difference between an .ost file and a set of personal folders is that the .ost file starts as a mirror image of your folders on the Exchange Server and is updated by the server during synchronization. A set of personal folders, on the other hand, is a storage location on your hard disk or a server other than the Exchange Server.

When you work offline, you work with the contents of an offline folder exactly as you do with a folder on a server. For example, you can change and move items in your offline Inbox, send messages that are placed in your offline Outbox, and read your offline public folders. You will continue to receive new messages in your mailbox, however, you will not be aware of these changes on the server until you connect to the network.

When you need to update the contents of the offline folders, you can synchronize the folder with the server and then continue to work offline. The Synchronize command connects to the server and copies the changes made. Any items that you delete from the offline folders or the corresponding server folder will be deleted from both after you synchronize the folders.


For additional information about using offline folders, click the article number below to view the article in the Microsoft Knowledge Base:

195435 OL2000: (CW) What Are Offline Folders and How Do You Use Them?

Additional query words: OL2K .ost grow limit maximum

Keywords: KB252430