An OST file (.ost) is an offline folder file in Microsoft Outlook. Offline folders make it possible for the user to work offline and then to synchronize changes with the Exchange server the next time they connect. The ability to work offline is useful in environments with limited or unreliable connectivity.
When Outlook is used in conjunction with Microsoft Exchange Server, you can set it to work with the Exchange mailbox offline. At that time, Outlook will make an exact copy of your mailbox on the Exchange Server, called offline folders, and store it in a local file, which is called offline folder file and has a .ost file extension. OST is the abbreviation for "Offline Storage Table".
When working offline, you can do everything with the offline folders just as if it were the mailbox on the server. For example, you can send emails which are actually put in the offline Outbox, you can also receive new messages from the other online mailboxes, and you can make changes to the emails and other items as you want.
However, all these changes will not be reflected to your mailbox on the Exchange server until you connect to the network again and synchronize the offline folders with the server.
During the synchronization process, Outlook will connect to the Exchange server via network, copy all the changes made so that the offline folders will be identical to the mailbox again. You can select to synchronize only a specific folder, a group of folders, or all the folders. A log file will be used to record all the important information about the synchronization, for your reference later.
Since Outlook 2003, Microsoft introduces a Cached Exchange Mode, which is actually an improved version of original offline folders. It is featured in more efficient synchronization mechanisms and more convenient offline operations.
Offline folders or Cached Exchange Mode has several advantages:
Make it possible for you to work with your Exchange mailbox even there are no network connections available.
When disaster happens on the Exchange server, such as server crashes, server database corruption, etc., the offline folder file on the local computer still contain a copy of your Exchange mailbox, with some offline updates. At that time, you can utilize Advanced Exchange Recovery to recover most of the contents in your Exchange mailbox by scanning and processing the data in local offline folder file.
The offline folder (.ost) file, like Outlook personal folders (.pst) file, is normally located in a predefined folder.
For Windows NT, 2000, XP and 2003 server, the folder is:
C:\Documents and Settings\
You may also search for file "*.ost" in your local computer to find the location of the file.
The OST file is the local copy of your Exchange mailbox, which contains all your most important personal communication data and information, including emails, folders, posts, appointments, meeting requests, contacts, distribution lists, tasks, task requests, journals, notes, etc.
When you have various problems with your mailbox or offline folders, for example, the Exchange server crashes or you cannot synchronize the offline updates with the server, we highly recommend you to use Advanced Exchange Recovery to recover all data in it.