The Ultimate Guide to PST Files: Everything You Need to Know

What Is a PST File?

A PST file — Personal Storage Table — is a local data file used by Microsoft Outlook to store your complete mailbox data on your computer. It is the backbone of email storage for hundreds of millions of Outlook users worldwide, holding everything from emails and contacts to calendar events and tasks in a single binary file.

Microsoft introduced the PST format with Outlook 97, and it has evolved through several versions since then. The format specification is publicly documented as part of Microsoft’s Open Specification initiative (MS-PST), though the internal structure is complex enough that working with PST files outside of Outlook requires specialized tools.

PST files are not simply containers of email text. They are structured databases that use B-tree indexing, property bags, and multiple allocation layers to organize potentially millions of items across thousands of folders. Understanding this structure is essential for anyone who manages, archives, or converts email data.

The Technical Architecture of PST Files

Three-Layer Design

The PST format is built on three interconnected layers:

1. NDB Layer (Node Database) The foundation of the PST file. This layer manages the physical storage, including the file header, block allocation, and B-tree indexes. Every piece of data in the PST is stored as a node in this layer, identified by a Node ID (NID).

2. LTP Layer (Lists, Tables, and Properties) Built on top of the NDB layer, the LTP layer provides two key abstractions:

• Property Context (PC) — A collection of key-value property pairs for a single item (like one email message)
• Table Context (TC) — A tabular structure for listing multiple items (like a folder’s contents)

3. Messaging Layer The highest layer, which defines the actual email objects: messages, folders, attachments, recipients, and their relationships. This is where concepts like “Inbox” and “Sent Items” are implemented.

ANSI vs. Unicode

PST files come in two variants:

Modern Outlook always creates Unicode PST files. You may still encounter ANSI PST files in older archives.

File Header

The first 564 bytes of a PST file contain the header, which includes:

• Magic number (offset 0): !BDN for Unicode PST, !BDN for ANSI PST (with different version bytes)
• File version (offset 10): Distinguishes ANSI (14-15) from Unicode (23+)
• Encryption type (offset 513): None (0x00), compressible (0x01), or high encryption (0x02)
• Root B-tree pointers: Locations of the Node BT and Block BT root pages
• CRC checksums: For header integrity verification

How Outlook Uses PST Files

Default Data File

When you configure a POP3 email account in Outlook, all downloaded messages are stored in a PST file. This file becomes the primary data store — if you delete it, your email is gone.

Archive Files

Outlook’s AutoArchive feature moves old messages from your primary mailbox to a separate archive PST file. This keeps your main mailbox lean while preserving older correspondence. You can also manually archive by exporting folders to PST.

Backup and Export

PST is Outlook’s standard export format. When you back up your mailbox, share email with a colleague, or prepare for a migration, the output is a PST file.

Shared and Delegated Access

In corporate environments, PST files are sometimes shared on network drives. While this works for small files, it is not recommended — PST files are not designed for concurrent access and can corrupt when accessed simultaneously over a network.

How to Open PST Files

With Microsoft Outlook (Windows)

1. Open Outlook
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 separate mailbox in the folder pane

With Microsoft Outlook (Mac)

Newer versions of Outlook for Mac can import PST files through File > Import. However, the process converts the PST to Outlook for Mac’s native format, and some metadata may not transfer perfectly.

Without Outlook

If you do not have Outlook or are on Linux, you have several options:

• Convert to a universal format — Convert PST to EML for individual messages readable by any email client, or convert PST to MBOX for Thunderbird-compatible archives
• Use a PST viewer — Several free and paid PST viewers exist for Windows, Mac, and Linux
• Use the readpst utility — Part of the libpst package on Linux, this command-line tool can extract messages from PST files

MailtoPst offers online PST conversion that works in any browser without software installation. Upload your PST file, choose the target format, and download the converted files.

PST File Size: Limits and Best Practices

Official Size Limits

Practical Size Recommendations

While Outlook supports PST files up to 50 GB, real-world performance degrades as files grow:

• Under 5 GB — Optimal performance, fast search and navigation
• 5-10 GB — Good performance for most users
• 10-20 GB — Noticeable slowdowns during search and startup
• 20-50 GB — Significant performance impact; Outlook may hang or crash
• Over 50 GB — Not supported; high risk of corruption

Best practice is to keep individual PST files under 10 GB. Split larger archives into multiple smaller files, organized by date range or department.

Reducing PST File Size

PST files do not shrink automatically when you delete messages. Deleted items remain in the file as “white space” until you compact the file:

1. Close all PST files in Outlook
2. Go to File > Account Settings > Account Settings
3. Select the Data Files tab
4. Select the PST file and click Settings
5. Click Compact Now

This process can take several minutes for large files.

Repairing Corrupted PST Files

Signs of Corruption

• Outlook cannot open the PST file
• Error messages about the file being damaged or in use
• Missing folders or messages
• Outlook crashes when accessing specific folders
• Search returns incomplete results

Microsoft’s Inbox Repair Tool (scanpst.exe)

Microsoft provides a built-in repair tool called scanpst.exe. Its location depends on your Outlook version:

• Outlook 2019/365: C:\Program Files\Microsoft Office\root\Office16\
• Outlook 2016: C:\Program Files (x86)\Office16\
• Outlook 2013: C:\Program Files (x86)\Office15\

To use it:

1. Close Outlook completely
2. Run scanpst.exe
3. Browse to the corrupted PST file
4. Click Start to begin the scan
5. If errors are found, click Repair

The tool creates a backup (.bak file) before attempting repairs. For severe corruption, you may need to run it multiple times.

When scanpst.exe Is Not Enough

For badly corrupted files, scanpst.exe may not recover all data. In these cases:

• Try creating a new PST file and importing what you can from the damaged one
• Use a third-party PST recovery tool
• Convert the recoverable portions using PST to EML to salvage individual messages

PST File Security and Encryption

Password Protection

PST files can be password-protected in Outlook. However, PST passwords provide minimal security — they are not real encryption. The password is stored as a CRC hash within the file itself and can be bypassed by numerous freely available tools. Never rely on PST passwords to protect sensitive data.

Encryption Levels

PST files support three encoding modes:

1. None (0x00) — No encoding; data stored in plain text
2. Compressible encryption (0x01) — A simple byte-substitution cipher; provides obfuscation but not real security. This is the default for new PST files.
3. High encryption (0x02) — A slightly stronger substitution cipher, but still not cryptographically secure

None of these encryption levels provide meaningful data protection by modern standards. If you need to secure email data, use filesystem-level encryption (BitLocker, FileVault) or encrypt the PST file with a dedicated encryption tool.

Protecting PST Files in Transit

When sharing or transferring PST files:

• Use encrypted file transfer (SFTP, HTTPS)
• Encrypt with a strong password using 7-Zip, WinRAR, or similar tools
• Never send unprotected PST files via email — they contain unencrypted personal data
• Use a GDPR-compliant conversion service like MailtoPst that encrypts data in transit and deletes files automatically

Converting PST Files

Why Convert?

PST files are locked into the Microsoft ecosystem. Converting them opens your email data to other platforms:

• Cross-platform access — PST files are primarily supported on Windows with Outlook
• Migration — Moving to Thunderbird, Apple Mail, Gmail, or other platforms
• Archiving — EML or MBOX formats are simpler for long-term storage
• Legal discovery — EML files are easier to review individually
• Data liberation — Freeing your data from vendor lock-in

Common Conversion Targets

What Gets Preserved During Conversion

A quality conversion tool preserves:

• All email headers (From, To, CC, BCC, Date, Subject, Message-ID)
• Message body (both plain text and HTML)
• All attachments with original filenames
• Folder structure and hierarchy
• Read/unread status and flags
• Embedded images and inline content
• Character encoding (UTF-8, ISO-8859-1, etc.)

Online Conversion with MailtoPst

MailtoPst provides online PST conversion that works directly in your browser:

1. Visit MailtoPst and select the source and target format
2. Upload your PST file
3. Wait for processing (typically a few minutes)
4. Download the converted files

All processing happens on GDPR-compliant EU servers. Files are automatically deleted after 24 hours. No software installation is required.

Managing PST Files in Enterprise Environments

Group Policy and PST Control

In corporate environments, IT administrators often need to manage PST file usage across the organization:

• Disable PST creation — Group Policy can prevent users from creating new PST files
• Limit PST file size — Registry settings control maximum file size
• Block PST files on network shares — Prevent the performance and corruption issues of network-stored PST files
• Centralize PST management — Tools exist to discover and catalog PST files across the network

Migration Away from PST

Many organizations are moving away from PST files entirely, centralizing all email in Exchange Online or Google Workspace. The migration path typically involves:

1. Discovering all PST files across the organization (workstations, file servers, network drives)
2. Cataloging and deduplicating the files
3. Importing PST content into cloud mailboxes or archives
4. Removing PST files from endpoints
5. Blocking future PST creation via Group Policy

Legal Hold and eDiscovery

PST files frequently appear in legal proceedings. Organizations subject to litigation hold must:

• Preserve all PST files that may contain relevant data
• Prevent modification or deletion of held files
• Index PST content for search and review
• Produce relevant messages in requested formats (often EML or PDF)

PST Files and GDPR Compliance

Data Subject Requests

Under GDPR, individuals can request access to or deletion of their personal data. PST files containing email are in scope:

• Right of access — You must be able to search PST files for a data subject’s communications
• Right to erasure — You may need to delete specific messages from PST files
• Right to portability — You may need to export a data subject’s emails in a portable format

Converting PST to EML format makes it easier to fulfill these requests, as individual messages can be searched, reviewed, and produced without opening the entire archive in Outlook.

Retention and Disposal

GDPR requires that personal data not be kept longer than necessary. PST files sitting on forgotten file servers or employee workstations can represent a data protection liability. Regular audits should identify and properly dispose of PST files that have exceeded their retention period.

Advanced PST Topics

PST File Merging

Merging multiple PST files into one is useful when consolidating archives. Outlook can import one PST into another, but for large-scale merging, dedicated tools are more efficient.

PST File Splitting

Splitting a large PST file into smaller files improves performance and manageability. You can split by:

• Date range (e.g., one file per year)
• Folder (e.g., one file per top-level folder)
• Size (e.g., maximum 5 GB per file)

PST and Unicode

Unicode PST files support all languages and character sets. If you are working with multilingual email data, ensure your PST files are in Unicode format (all modern Outlook versions create Unicode PSTs by default) and that your conversion tool handles Unicode correctly.

Recovering Deleted Items from PST

When you delete a message in Outlook, it goes to the Deleted Items folder. When you empty Deleted Items, the data is not immediately erased — it becomes recoverable “soft-deleted” data. Specialized recovery tools can sometimes retrieve these items from the PST file’s unallocated space.

Frequently Asked Questions

What is the maximum PST file size?

The Unicode PST format supports files up to 50 GB by default. This can be increased through Windows registry settings, but Microsoft recommends keeping PST files under 50 GB for performance and reliability. Practically, files under 10 GB perform best.

Can I open a PST file on Mac?

Newer versions of Outlook for Mac can import PST files, but the process is not always reliable for large or complex archives. For guaranteed access on Mac, convert PST to MBOX for use with Thunderbird, or convert PST to EML for universally readable individual messages.

Is a PST file the same as an OST file?

No. While they share a similar internal structure, PST files are standalone archives you can move between computers, while OST files are cache files tied to a specific Outlook profile and Exchange/Office 365 account. If the account is removed, the OST becomes inaccessible without conversion to PST.

How do I password-protect a PST file?

In Outlook, right-click the PST file in the folder pane, select Data File Properties > Advanced > Change Password. However, PST passwords are extremely weak — they provide no real security. Use filesystem encryption or a proper encryption tool for genuine data protection.

Can I access a PST file without Outlook?

Yes. You can use free PST viewers, the readpst command-line tool on Linux, or online conversion services like MailtoPst to convert PST to EML or convert PST to MBOX without installing Outlook.

How do I repair a corrupted PST file?

Start with Microsoft’s Inbox Repair Tool (scanpst.exe). Close Outlook, run the tool, point it at your PST file, and let it scan and repair. For severe corruption, you may need to run it multiple times or use a third-party recovery tool. Always back up the file before attempting repairs.

What happens if my PST file exceeds the size limit?

If a PST file reaches its size limit, Outlook will stop saving new messages to it. You may see errors about the file being full. The solution is to archive older messages to a new PST file, delete unnecessary items, and compact the file to reclaim space.