The type of e-mail account you use primarily determines how your e-mail messages, calendar, and other items are delivered and stored.
Microsoft Exchange Server e-mail account
If you use Microsoft Exchange Server, typically your e-mail messages, calendar, and other items are delivered to and stored on the server.
You can also work offline or use Cached Exchange Mode. A local copy of your items is kept on your computer in an Offline Folder file (.ost). The .ost file is synchronized with the Exchange server. Offline Folder files allow you to work with your items when a connection to the Exchange server may not be possible or wanted.
Your items can also be moved or archived to a Personal Folders file (.pst). Since a .pst file is kept on your computer, it is not subject to mailbox size limits on the server. Outlook can be configured to deliver new items to a .pst file, but doing so has several disadvantages including not being able to work with your items from other computers or when using Microsoft Outlook Web Access with your Exchange e-mail account.
POP3, IMAP, and HTTP e-mail accounts
If you don't use an e-mail account on an Exchange server, your e-mail messages, calendar, and other items are delivered to and stored locally in a Personal Folders file (.pst) called Outlook.pst. Outlook.pst is the default data file name.
In Microsoft Office Outlook 2003, a new Personal Folders file format, called Microsoft Outlook Personal Folders file (.pst) is introduced that offers greater storage capacity for items and folders and supports multilingual Unicode data. A file created in the Microsoft Outlook Personal Folders file (.pst) format is not compatible with earlier versions of Outlook and cannot be opened in a previous version of Outlook.
To create a data file that is compatible with earlier versions of Outlook, use the Microsoft Outlook 97-2002 Personal Folders file (.pst) format. This file format is the same as the format that was available in earlier versions of Outlook. It does not support multilingual Unicode data and offers the same storage capacity for items and folders that was available in earlier versions.
To see where default store and additional data files are kept on different versions of Microsoft Windows, see the "Outlook file locations" topic. Keep these points in mind:
Only data files that you created will appear in the Outlook Data Files dialog box (File menu, Data File Management command). Your default data file, Outlook.pst, does not appear in the list.
Note If your Windows user account is not password-protected or other people have access to your Windows user account, do not use the Save this password in your password list option.
Use a data file to organize or back up items
Use a data file to distribute information
You can use a data file to make information, such as Contacts, available to another computer you use or to people you work with. The easiest way to do this is to export (copy) the information to a .pst file using the Import and Export wizard.