What is a PST file?

When cleaning up disk space, tech users often come across files with strange, apparently meaningless extensions – such as pst or odt. It becomes even more confusing when the files cannot be opened simply by double-clicking them, and this is because they are in a program-specific format. One of these files is the PST file in Outlook. A PST file can normally only be opened with Outlook and represents a backup copy for user-related Outlook data. Especially if you store your data locally on the PC rather than on the Outlook server, PST files are an important file format to know about.

PST stands for Personal Storage. So, a PST file is a personal data container that you can think of as a cross between a safe and a file folder. In the PST file, Outlook collects personal, synchronised user data, and stores it as a backup copy on the computer. PST is a Microsoft proprietary file format, i.e. it is licensed by Microsoft and usually can be opened only with Outlook and a corresponding Outlook account.

The role PST files play for users depends primarily on the type of Outlook account:

• Outlook with POP accounts (also POP3): you download emails directly from the mail server and store them locally on the computer in PST format.
• Outlook with IMAP, Outlook 365, Exchange, or Outlook.co.uk accounts: ‘original data’ remains on the mail server, while offline Outlook data files, the OST files, are cached on the device.

The advantage of the PST format is its storage size compared to the much smaller storage volume of mailboxes. As of Office 2010, PST files have a storage size of 50 gigabytes. With PSTs you can:

• Archive emails with Outlook in a local PST file, e.g. distinguished into private and business message archiving
• Save backup copies of various personal user data as an Outlook backup locally on your computer
• Export data from your Outlook account as a PST file to free up space in your mailbox

Outlook’s great strength is that it synchronises various data and unites them on one interface. It also creates links between often separate areas such as mailboxes, calendars, and contact lists. A loss of data would be particularly unfortunate in this case. To protect coordinated user data from hardware errors or to facilitate import and export from one computer or network to another, Outlook creates backup copies in PST format.

This not only makes it easier for Outlook and its users to access the data, but also makes it possible to organise the ‘personal storage’, separate PST files into private and business matters, and separate them from linked apps or outlook alternatives

Archiving user data is done by moving the data and its folder structures to local PST files. You can decide whether you want to keep the archived data in the mailbox or delete it automatically. In addition, you can specify whether you want to perform archiving automatically or manually at intervals.

• Automatic archiving: Go to ‘File’, then to ‘Options’ and to ‘Advanced’. In the ‘AutoArchiving’ dialog you can activate automatic archiving under ‘AutoArchiving every x days/weeks/months’. You determine when to delete archived data from the mailbox and where to store PST files. Under ‘Do not activate items in this folder’ you also define which specific files you want to archive.
• Manual archiving: Instead of automatic archiving, you can also move data manually to a PST file. To do this, go to ‘Tools’ under ‘File’ (for older versions: ‘Cleanup tools’) and select the ‘Clean up old items’ command. This allows you to define archiving by age of the files and select all or only specific folders.

Where a PST file is depends on three factors:

• Outlook version
• Outlook account type (POP or IMAP, Microsoft 365, Exchange, or Outlook.co.uk)
• Automatic or user-defined storage location

The following locations can be used for a PST file:

• Windows 10: :\user\\application data\local\microsoft\outlook or :\user\\roaming\local\microsoft\outlook.
• Older versions of Windows: :\Documents and Settings\\Local Settings\user\application data\local\microsoft\outlook

The automatic location of PST files from Outlook 2013 or 2016 is usually the Documents/Outlook Files folder. Files from older or upgraded versions of Outlook 2007 or earlier can be found in the hidden folder <drive>:\user\<username>\AppData\Local\Microsoft\Outlook. To really find all backup files, you should display hidden files and folders

The fastest way to find PST files is directly with Outlook. To do this, go to ‘File’, select ‘Account Settings’ under ‘Account Settings’ and find the location of the Outlook file in question by clicking ‘Open Data Location’.