What Is a PST File? How to Open, View, and Use Outlook PST Files

PST stands for Personal Storage Table. It is the file format Microsoft Outlook uses to store email messages, calendar entries, contacts, tasks, and notes on a local hard drive. If you have ever opened Outlook on a Windows PC, there is almost certainly a .pst file sitting somewhere on that machine.

People encounter PST files in a few common situations: IT admins create them during email migrations, Microsoft 365 generates them when exporting eDiscovery results, and individual users create them manually to archive old mail. The format dates to the mid-1990s, so it is widespread, but that age comes with quirks worth understanding before you try to open one.

What a PST File Contains

A single PST file can hold a surprising amount of data. Outlook uses it as a container for all of the following:

• Email messages, including all attachments
• Calendar events and appointments
• Contacts and distribution lists
• Tasks and to-do items
• Notes
• Journal entries

Internally, the file is organized as a hierarchical tree of folders that mirrors the folder structure you see in Outlook's navigation pane. A PST can represent a single email account's inbox, a subset of messages exported by date range, or an entire archive covering years of correspondence. The scope depends entirely on how and when it was created.

PST files are binary files in a proprietary Microsoft format. You cannot open one in a text editor and read the contents. You need either Outlook or a dedicated PST viewer to see what is inside.

How PST Files Are Created

There are three main ways a PST file ends up in someone's hands.

Outlook's Export Tool

In any version of Outlook for Windows, go to File > Open & Export > Import/Export and choose "Export to a file." Selecting "Outlook Data File (.pst)" creates a local copy of any folder or your entire mailbox. This is how individuals create personal backups and how IT departments package mailbox data during migrations or offboarding.

Microsoft 365 eDiscovery and Content Search Export

When legal or compliance teams run a search in the Microsoft Purview compliance portal (formerly the Microsoft 365 compliance center), one of the standard output formats is PST. The export tool packages responsive email into one or more PST files, along with a mapping file. This is one of the most common ways PST files appear in legal matters.

Exchange Mailbox Export via PowerShell

Exchange administrators can export individual mailboxes directly to PST using the New-MailboxExportRequest PowerShell cmdlet. This is common during employee offboarding, legal holds, and HR investigations where a mailbox needs to be preserved but the account itself may be closed.

Where PST Files Are Stored on Your Computer

Outlook saves PST files in a default location, though users can change this. On Windows 10 and 11, the typical path is:

C:\Users\[username]\Documents\Outlook Files\

On older Windows installations, it is often:

C:\Users\[username]\AppData\Local\Microsoft\Outlook\

The AppData folder is hidden by default, so you may not see it in File Explorer without enabling hidden items. The most reliable way to find your PST file location from within Outlook is to go to File > Account Settings > Account Settings, click the Data Files tab, and look at the path listed for each entry.

On a Mac, Outlook does not use PST files. It uses its own .olm format or stores data in an internal database. PST is a Windows-only format. If someone sends you a PST from a Windows machine and you are on a Mac, you need a third-party tool to access it.

How to Open a PST File in Outlook

If you have a Windows machine with Outlook Classic (the traditional desktop application, not the web-based "new Outlook") installed, opening a PST is straightforward.

1. Open Outlook Classic
2. Go to File > Open & Export > Open Outlook Data File
3. Browse to the .pst file and select it
4. The file appears as a new folder tree in the left navigation pane

From there you can search, read, copy, or move messages just as you would with your regular mailbox. When you are done, right-click the PST folder in the navigation pane and choose "Close [filename]" to disconnect it.

One important caveat for Microsoft 365 users: the "new Outlook" app that Microsoft has been rolling out does not support PST files at all. You need Outlook Classic to work with PST directly. If you are not sure which version you have, check whether your Outlook has a toggle in the top-right corner labeled "New Outlook" or "Try the new Outlook." If the toggle is active and you are in the new version, you will need to switch back to Classic before you can open a PST.

How to Open a PST File Without Outlook

Outlook is not your only option. Several free and paid tools can read PST files without a Microsoft Office license, which matters if you are on a Mac, if your organization does not have Outlook licenses available, or if you need to view a PST file outside of the Outlook environment for review purposes.

A few things to keep in mind when evaluating these options. Free desktop viewers like Kernel and SysTools let you browse and preview message content but typically restrict export to their paid tiers. Online viewers are convenient for a quick look at a small file, but uploading a PST containing sensitive legal or business communications to a third-party web service carries obvious privacy considerations. For anything involving an active legal matter, a purpose-built review platform is almost always the better path.

PST vs OST: What Is the Difference?

Outlook uses two local file formats, and they are easy to confuse because both have similar names and both live on your hard drive.

A PST file (Personal Storage Table) stores data you explicitly created or exported. It is a standalone archive that you can move, share, copy, or back up independently of any email account.

An OST file (Offline Storage Table) is a local synchronization cache that Outlook creates automatically when you connect an Exchange or Microsoft 365 account. It mirrors your server-side mailbox so Outlook can function without an internet connection. OST files are not designed to be portable. They are tied to a specific account and Outlook profile, and you cannot simply copy one to another machine and open it. If you receive an .ost file and need to access its contents, you typically need to convert it first using a PST converter tool, many of which handle OST-to-PST conversion as a primary use case.

Outlook also generates NST files for shared Group calendars, but these are far less common and generally not relevant outside of specific Microsoft 365 Group setups.

PST File Size Limits

PST files are not infinitely scalable, and the size ceiling has changed across Outlook versions.

• Outlook 97 through 2002: ANSI format, 2 GB hard maximum. Files that hit this limit became corrupt, sometimes silently.
• Outlook 2003 and later: Unicode format, default limit of 20 GB, configurable via registry up to 50 GB.
• Current versions: The default performance warning threshold is around 10 GB. Files can grow to 50 GB, but search and load times degrade significantly past that point.

A PST approaching its limit slows Outlook down and becomes increasingly susceptible to corruption. If Outlook has ever frozen while you tried to search through old mail, an oversized PST is one of the first things to check.

In eDiscovery contexts, large PST files carry additional complications. A 30 GB PST exported from Microsoft 365 may take hours to process through a review platform, and the per-GB fees on some platforms add up quickly. Some legal teams specifically request that opposing parties split PST productions into smaller files (typically under 5 GB) to make processing more manageable.

PST Corruption and Recovery

PST files are susceptible to corruption, particularly older ANSI-format files, but Unicode files are not immune. The most common causes are Outlook closing unexpectedly (due to a crash or power loss during a write operation), PST files stored on a network share getting an interrupted write, and files that have grown too large for the format to handle cleanly.

Microsoft ships a repair tool called ScanPST.exe (also called the Inbox Repair Tool) with every version of Outlook. On a standard Office 365 installation, it lives at:

C:\Program Files\Microsoft Office\root\Office16\SCANPST.EXE

The exact path varies depending on whether you have a 32-bit or 64-bit Office installation and which version you are running. Running ScanPST on a corrupted PST scans the file for structural errors and attempts to rebuild its internal tables. It is free and works well for minor corruption. For serious damage where ScanPST fails or reports extensive errors, third-party tools like Stellar Repair for Outlook or Kernel for Outlook PST Repair generally have a stronger track record for recovering message content from badly damaged files.

One thing ScanPST does not do: recover permanently deleted items. It repairs the file structure, not deleted message content. If messages were deleted and the .pst was subsequently compacted, those messages are gone from the PST regardless of what repair tool you use.

PST Files in eDiscovery

PST is one of the most common output formats in email eDiscovery, which means legal teams encounter them constantly. When Microsoft 365's Purview eDiscovery export tool runs, the default package includes one or more PST files along with a mapping CSV that identifies which PST contains which custodian's data. Large matters can result in dozens of PST files totaling hundreds of gigabytes.

This creates several practical challenges for review teams. PST files must be processed and ingested into a review platform before the content is searchable. You cannot keyword-search a raw PST file the way you can a folder of PDFs. Processing time and platform costs scale with file size and volume, so large PST productions can be expensive to work through.

There is also a metadata concern. The PST export and re-import cycle does not always preserve all message metadata cleanly. Thread context, attachment linkages, and certain header fields can degrade. For this reason, some practitioners request that email productions be made in EML or MSG format rather than PST, since those formats represent individual messages rather than a single monolithic container and tend to map more cleanly to standard eDiscovery load files. Whether you can make that request depends on what the producing party's system supports and what the parties have agreed to in an ESI protocol.

PST files also appear on the receiving end of legal holds. If a custodian has years of email stored in a local PST archive rather than on the server, that file needs to be collected and preserved as part of the hold. PSTs sitting on local hard drives are easy to miss in a custodian interview, and they fall outside the reach of server-side preservation tools. Asking custodians directly whether they have ever exported their email to a .pst file is a step that gets skipped more often than it should.

PST as an Archival Format: What You Should Know

PST was designed in the 1990s as a local mailbox file, not as a long-term archive format. It has held up well given its age, but it comes with real risks if you treat it as a permanent home for important data.

The format is proprietary and tightly coupled to the Outlook ecosystem. Microsoft has published technical specifications for the PST file structure, but the format remains essentially tied to Outlook. The "new Outlook" application Microsoft has been rolling out as the default in Windows removes PST support entirely. If that trend continues, organizations with years of email stored in PST archives will need a conversion strategy before they lose access to Outlook Classic.

The format is also fragile by nature. A single binary file housing gigabytes of email is a single point of failure. A disk error, an interrupted network copy, or a graceless shutdown during a write can render the entire archive unreadable. There is no easy way to repair partial corruption and recover just the damaged section; you either get the whole file or you spend time and money on recovery tools hoping you get most of it.

For anything meant to last more than a few years, PST files are better treated as a transport format than a destination. Ingesting them into a more durable system, whether a purpose-built compliance archive, an eDiscovery platform with long-term storage, or even individual EML or MSG files on managed, backed-up storage, is more reliable than leaving a 20 GB .pst file on a file share and hoping nothing goes wrong with it before someone needs it.