When you use Microsoft Outlook, you need a place to keep your e-mail messages, calendar, tasks, and other items. This storage place, known as a data file, allows you to keep your data on your computer.
When saving your items on your computer, Outlook uses a data file called Outlook Personal Folders file (.pst). If you are using a Microsoft Exchange account, your items are usually delivered to and saved on the e-mail server. To allow you to work with your messages even when you can't connect to the e-mail server, Outlook offers offline folders, which are saved in an Offline Folder file (.ost) on your computer.
The primary distinctions between the two Outlook data file types are:
- Outlook .ost files are used only when you have an Exchange account and chose to work offline, or you use Cached Exchange Mode.
- Outlook .pst files are used for POP3, IMAP, and HTTP accounts. When you want to create archives or backup files of the Outlook folders and items on your computer, including Exchange accounts, you must create and use additional .pst files.
In another word, when saving your items on your computer, Outlook uses a data file called Outlook Personal Folders file (.pst). If you are using a Microsoft Exchange account, your items are usually delivered to and saved on the e-mail server. To allow you to work with your messages even when you can't connect to the e-mail server, Outlook offers offline folders, which are saved in an Offline Folder file (.ost) on your computer.
Outlook Data Files (.pst) are saved on your computer in the Documents\Outlook Files folder.
Offline Outlook Data Files are saved in the drive:\Users\user\AppData\Local\Microsoft\Outlook folder. It isn’t necessary to back up an offline Outlook Data File (.ost) as it is a copy of the information on the server. If you set up the account again or on another computer or device, a synchronized copy of your messages are downloaded from the server.