What is an Outlook Data File? Why does it have an extension of .pst?

An Outlook Data File, also called Personal Folders File, is a data file that can be used to store email messages, contact entries, appointments, tasks, notes, and journal entries from Microsoft Exchange Client, Windows Messaging, and Microsoft Outlook.

An Outlook Data File uses the Personal Storage Table (PST) open proprietary file format. This is why Outlook Data Files are using .pst file extensions.

Here is a good introduction on Personal Folders File from Microsoft: "A Personal Folders file (.pst) is an Outlook data file that stores your messages and other items on your computer. Personal Folders files are the most common format in which information in Outlook is saved by home users or in small organizations. Home users usually use an Internet service provider (ISP) to connect to the Internet. The ISP also provides one or more e-mail accounts. The most common types of accounts are referred to by their Internet protocol names - POP3 and IMAP or just POP and IMAP. Another type of account is an HTTP or Web-based account that works similar to IMAP e-mail accounts. All three account types use a .pst file. Your items can also be moved or archived to a Personal Folders file (.pst). Because a .pst file is kept on your computer, it is not subject to mailbox size limits on the mail server. By moving items to a .pst file on your computer, you can free up storage space in the mailbox on your mail 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 when you are using Microsoft Outlook Web Access with your Exchange Server e-mail account or when you are working on another computer."

⇒ Using outlook.office365.com

⇐ What Is Personal Storage Table (.pst) File

⇑ General Questions on Microsoft Outlook

⇑⇑ MS Outlook - Frequently Asked Questions