Outlook Data Files

For most Outlook users, program data and items are stored in an Outlook Personal Folders file. This is true if you are using a POP, an IMAP, or an HTML e-mail account. The file has the .pst extension and is by default named Outlook.pst. The folder on your hard disk where this file is normally kept is as follows:

• Windows XP and Windows Server 2003: X:Documents and SettingsuserLocal SettingsApplication DataMicrosoftOutlook

• Windows Vista: X:userLocal SettingsAppDataMicrosoftOutlook

Here, X is the drive letter, usually C, where the operating system stores user settings, and user is the name you have used to log onto Windows. If the computer is configured for more than one user, each will have his or her own separate and independent Outlook Personal Folders file.

You can have more than one Personal Folders file, but only one is designated as the default, which means that Outlook uses it to store account settings, messages, and other items. Additional PST files are used for special purposes such as archiving old items. You cannot change the storage location of the default PST file.

Outlook data file compatibility

Beginning with Outlook 2003 and continuing with the current version, Microsoft changed the internal format of PST files to allow for storage of more items and folders and to support multilingual Unicode data. This format is not compatible with Outlook versions 97 through 2002. If you install a new Outlook over one of these older versions, the old format PST file is automatically converted to the new format. If, however, you want your PST file to be compatible with Outlook 2002 and earlier, you must create a PST file in the older format.

Offline folders file

If you use a Microsoft Exchange e-mail account rather than, or in addition to, an IMAP, a POP, or an HTML account, you may have an Offline Folders file (which has the .ost extension). Normally, Exchange keeps copies of your messages and other items on the server, but you can configure Outlook to keep a local copy of the items on your system, in the Offline Folders file. Doing so allows you to work with your Outlook items when a connection to the Exchange server is not available.