When you use Microsoft Outlook on a Windows computer, your email messages, calendar, tasks, and other items are saved on a mail server, on your computer, or both. Outlook items that are saved on your computer, are kept in an Outlook Data File (.pst and .ost).
About Outlook Data Files (.pst and .ost)
There are two types of Outlook Data Files used by Outlook. An Outlook Data File (.pst) is used for most accounts. If you are using a Microsoft Exchange account, your items are usually delivered to and saved on the mail server. To allow you to work with your messages even when you can’t connect to the mail server, a second type of data file that is named an offline Outlook Data File (.ost) is kept on your computer.
The primary differences between the two types of Outlook data files are as follows:
Outlook Data Files (.pst) are used for POP3, IMAP, and web-based mail accounts. When you want to create archives or back up your Outlook folders and items on your computer, such as Exchange accounts, you must create and use additional .pst files.
Outlook Data Files (.ost) are used when you have an Exchange account and want to work offline or use or use the default Cached Exchange Mode. Outlook Data Files (.ost) are always copies of items that are saved on a mail server and don’t have to be backed up like Outlook Data Files (.pst).
Outlook Data File (.pst)
A Personal Folders file (.pst) is an Outlook data file that stores your messages and other items on your computer. Because a .pst file is kept on your computer, it is not subject to mailbox size limits on the mail server. By moving items to a .pst file on your computer, you can free up storage space in the mailbox on your mail server. Outlook can be configured to deliver new items to a .pst file, but if you do this, it has several disadvantages. This includes being unable to work with your items when you are accessing it through the web browser or when you are working on another computer.
Offline Outlook Data File (.ost)
Typically, when you use a Microsoft Exchange Server account, your email messages, calendar, and other items are delivered to and saved on the server. You can configure Outlook to keep a local copy of your items on your computer in an Outlook data file that is named an offline Outlook Data File (.ost). This allows you to use Cached Exchange Mode or to work offline when a connection to the Exchange computer may not be possible or wanted. The .ost file is synchronized with the Exchange computer when a connection is available.
Offline folders are replicas of the folders found in your mailbox on the computer that is running Microsoft Exchange. They make it possible to take a folder from a server location, work with the contents of the folder when you are not connected to the network, and then, when you are connected again, update the folder and its corresponding server folder to make the contents of both folders identical. This process is known as synchronizing folders.
You can add, delete, and change the contents of an offline folder exactly as you can for a folder on a server. For example, you can change and move items between folders, send messages that are included in your offline Outbox, and view the contents of your offline public folders. Meanwhile, new messages are kept in your Inbox on the server, and other people might add, delete, and change items in public folders. You’ll not be aware of these changes on the server until you synchronize.
Outlook Data Files (.pst) File Location
The fastest way to open the folder where your Outlook Data File (.pst and .ost) is saved is to do the following:
- For Windows Outlook clients, click the File tab.
- Click Account Settings, and then click Account Settings.
- On the Data Files tab, click an entry, and then click either Open File Location or Open Folder Location.
- The folder containing your .pst will now open.
Outlook Data File (.ost) File Location
The .ost file is synchronized with the items on the server that runs Exchange. Because your data remains on the Exchange server, you can re-create this .ost file on your new computer without having to back up the .ost file.
Windows Vista, 7 and 8: drive:\Users\<username>\AppData\Local\Microsoft\Outlook
Mac OS X:
Additional information on the location of your Outlook Data Files can be found here.