Outlook Data Files (.pst) are used for POP3, IMAP, and web-based mail accounts.The primary differences between the two types of Outlook data files are as follows: To allow you to work with your messages even when you cannot 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.
If you are using a Microsoft Exchange account, your items are usually delivered to and saved on the mail server. An Outlook Data File (.pst) is used for most accounts. There are two types of Outlook data files used by Outlook for Windows.
