What Does PST Stand For?
PST stands for Personal Storage Table. It is the file format Microsoft Outlook uses to store local copies of email messages, calendar events, contacts, tasks, and notes. When you archive your Outlook mailbox or create a backup, Outlook writes that data into a PST file with the .pst extension.
The PST format has been a core part of the Microsoft Outlook ecosystem since Outlook 97. Over the years, it has evolved from the older ANSI format (with a 2 GB size limit) to the current Unicode format (supporting up to 50 GB by default in modern Outlook versions).
What Is Inside a PST File?
A PST file is a structured binary database, not a simple text file. It contains multiple types of data organized in a hierarchy:
- Emails — full message content including subject, body (plain text and HTML), headers, and all attachments.
- Folders — the complete folder tree: Inbox, Sent Items, Drafts, Deleted Items, Junk Email, and any custom folders you created.
- Contacts — names, email addresses, phone numbers, physical addresses, and other contact fields.
- Calendar events — appointments, meetings, recurrence patterns, attendees, and reminders.
- Tasks and notes — items from Outlook’s task management and sticky notes features.
- Journal entries — activity logs if the journal feature was enabled.
Technical Architecture
Under the hood, a PST file follows the MS-PST Open Specification published by Microsoft. The format has three layers:
- NDB (Node Database) layer — manages raw data blocks, B-tree structures, and the file header.
- LTP (Lists, Tables, and Properties) layer — organizes data into Property Contexts (for individual items) and Table Contexts (for folder listings).
- Messaging layer — maps email-specific concepts like message stores, folders, and message objects onto the LTP structures.
This layered design makes PST files efficient for storing millions of items but also makes them difficult to read without dedicated parsing software.
How Are PST Files Created?
PST files are generated in several ways:
- Manual archive — Outlook users can create a PST file through File, then Info, then Account Settings, then Data Files, then Add.
- AutoArchive — Outlook’s AutoArchive feature moves old messages to a PST file on a schedule.
- Export — using File, then Open & Export, then Import/Export to export a mailbox to a PST file.
- Backup tools — third-party backup solutions often export Outlook data to PST.
- IT migration — administrators export Exchange mailboxes to PST for migration or compliance purposes.
Where Are PST Files Stored?
The default location depends on the operating system and Outlook version:
- Windows 10/11:
C:\Users\<username>\Documents\Outlook Files\ - Older Windows:
C:\Users\<username>\AppData\Local\Microsoft\Outlook\
You can also store PST files on network drives, external hard drives, or USB storage, though Microsoft recommends keeping them on a local drive for performance.
How to Open a PST File
With Outlook
Open Outlook, go to File, then Open & Export, then Open Outlook Data File, and browse to the PST file. It appears as a separate folder tree in your Outlook sidebar.
Without Outlook
If you do not have Outlook installed, upload the PST to mailtopst.com to browse its contents online or convert it to a universal format like EML or MBOX. See the detailed guide: How to open a PST without Outlook.
Common PST File Issues
- Corruption — PST files can become corrupted after a crash, improper shutdown, or when they grow very large. Microsoft provides the ScanPST.exe tool (Inbox Repair Tool) for basic repair. See How to convert a corrupted PST.
- Size limits — ANSI PST files (Outlook 2002 and earlier) are limited to 2 GB. Unicode PST files support up to 50 GB, but performance degrades above 10 GB.
- Password protection — PST files can be password-protected, though this protection is considered weak by security standards. See How to convert a password-protected PST.
Frequently Asked Questions
Is a PST file the same as an OST file?
No. A PST is a portable archive that any Outlook installation can open. An OST is a local cache tied to a specific Exchange or Microsoft 365 account. See PST vs OST for a full comparison.
Can I open a PST file on a Mac?
Outlook for Mac can open PST files, but if you do not have Outlook, you can convert the PST to EML or MBOX using MailtoPst and open the result in Apple Mail. Try free — no credit card required.
How do I reduce the size of a PST file?
Delete unnecessary emails and empty the Deleted Items folder. Then compact the PST file through Outlook: right-click the PST in the folder list, select Data File Properties, then Advanced, then Compact Now.
Can PST files contain viruses?
The PST file itself is not executable, but it can contain email attachments that are malicious. Always scan attachments with antivirus software before opening them.
What is the maximum size of a PST file?
Unicode PST files (the default since Outlook 2003) can theoretically reach 50 GB. However, Microsoft recommends keeping PST files under 10 GB for optimal performance.
Upload your file now to explore or convert your PST archive.