If your e-mail is delivered to a Microsoft Exchange Server account, you'll be able to tell because you'll see the word "Mailbox" next to the Outlook Today folder. Messages that appear in this folder or its subfolders are stored on the server. In contrast, messages in an Archive Folders or Personal Folders file are stored locally in a .pst file on your own computer.
It may seem as if your e-mail is already on your computer because that's where you look at it. However, if you use a Microsoft Exchange Server e-mail account or Web-based e-mail service, most of your e-mail lives on an e-mail server.
Suppose you want to reduce the size of your mailbox by storing your messages on your own computer, or you just want to back up some messages related to an important project. How could you get that e-mail onto your own computer? And once you did move it, where would that e-mail be stored?
The answer is, you'd store it in an Outlook data file called a Personal Folders file. Simply speaking, it's a file that stores your e-mail messages and other Outlook items, such as appointments and contacts. A Personal Folders file is also referred to as a personal store, PST, or .pst file.
In Microsoft Word, for example, when you create a document, that document is saved as a document file (.doc) that's stored on your computer. Similarly, Outlook lets you create data files for storing e-mail messages, contacts, appointments, and so on. An Outlook file of this type is called a personal store (.pst) because you personally store it on your computer. And just as Word document files can be opened, closed, moved, or copied, so can Outlook .pst files.