PST & OST: Complete Guide to Microsoft Outlook Data Files
For millions of Microsoft Outlook users, the PST file is the unsung keeper of their digital work life — storing years of emails, contacts, calendar appointments, and tasks in a single binary file on their hard drive. PST files are reliable enough that many people never think about them until something goes wrong. When it does, understanding what PST and OST files are, how they work, and how to repair, export, or migrate them becomes urgent.
What Is a PST File?
PST (Personal Storage Table) is Microsoft Outlook's local data file format. It stores a complete mailbox — email messages, folders, contacts, calendar items, tasks, notes, and journal entries — in a single proprietary file on your local disk.
PST files are used for:
- Local email archiving: storing old emails offline to keep the mailbox size manageable
- Standalone (non-Exchange) accounts: POP3 email accounts store all mail in a PST file
- Personal archive: deliberately moving items out of the server mailbox for long-term retention
Two Generations of PST Format
ANSI PST (Outlook 97-2002): The original PST format, limited to 2 GB maximum. Files that approach this limit corrupt silently as Outlook continues writing past the safe threshold. If you have a very old PST file, this is the version you have.
Unicode PST (Outlook 2003 and later): The modern format, supporting files up to 50 GB by default (configurable up to 100 GB via registry). All new PST files created by Outlook 2003+ use Unicode format. This is the format you should be using.
Default PST File Location
On Windows 10/11 with Outlook 2016+:
C:\Users\[username]\AppData\Local\Microsoft\Outlook\[email-address].pst
On older Windows versions or Outlook 2010:
C:\Users\[username]\Documents\Outlook Files\[email-address].pst
Find your PST location in Outlook: File → Account Settings → Account Settings → Data Files tab.
What Is an OST File?
OST (Offline Storage Table) is the local cache file that Outlook creates when connected to an Exchange Server, Microsoft 365, or an IMAP account. Unlike PST (which is the primary storage), OST is a synchronized mirror of your server mailbox:
- OST is automatically managed by Outlook: it syncs changes between the server and local cache
- You cannot add an OST to another Outlook installation: OST files are tied to the specific Exchange account that created them
- Deleting the OST is safe if you have server access: Outlook will rebuild it by re-syncing from the server
- If you lose server access, your OST becomes your only copy — treat it carefully
OST File Location
C:\Users\[username]\AppData\Local\Microsoft\Outlook\[email-address].ost
PST vs OST: Key Differences
| Feature | PST | OST |
|---|---|---|
| Primary use | Local archive / POP3 mail | Exchange/Microsoft 365 cache |
| Data authority | PST is the master copy | Server is the master copy |
| Portability | Can be opened on any Outlook | Tied to originating account |
| Size limit | 50 GB (Unicode format) | 50 GB (same format internally) |
| Rebuilding | Cannot be rebuilt — data loss if lost | Rebuild by re-syncing from server |
| When server unavailable | Primary offline storage | Read/write locally, sync when reconnected |
| Export | Manually managed | Automatically managed by Outlook |
Common PST Problems and Fixes
Corrupt PST File
Corruption causes: abrupt Outlook termination, disk errors, antivirus scanning open PST, PST approaching the 2 GB ANSI limit.
ScanPST.exe (Inbox Repair Tool) is included free with every Outlook installation:
Location:
- Outlook 2019/Microsoft 365:
C:\Program Files\Microsoft Office\root\office16\SCANPST.EXE - Outlook 2016:
C:\Program Files (x86)\Microsoft Office\Office16\SCANPST.EXE
Using ScanPST:
- Close Outlook completely
- Run SCANPST.EXE
- Browse to your PST file location
- Click Start to scan
- If errors found, click Repair
- ScanPST creates a backup (.bak) before repairing
For severely corrupt files where ScanPST fails, third-party tools like Stellar Repair for Outlook, Kernel for Outlook PST Repair, or DataNumen Outlook Repair may recover more data.
PST File Too Large
If your PST is approaching 50 GB or if Outlook performance has slowed:
- Create a new PST archive: File → New → Outlook Data File → choose a different name
- Move old email: select old folders or date-filtered items → Move to Folder → choose the new PST
- Use AutoArchive: Outlook's built-in tool to automatically move old items to an archive PST based on age (File → Options → Advanced → AutoArchive Settings)
- Compact the PST: after deleting items, reclaim space with File → Account Settings → Data Files → Settings → Compact Now
Cannot Open PST (Access Denied)
This occurs when antivirus software has locked the file, another application is using it, or file permissions are incorrect. Try: reboot, disable real-time antivirus scanning for the Outlook folder, or take ownership of the file (Windows Security → Properties → Security tab → Advanced → Change owner).
Exporting PST Contents
Export to EML (Individual Emails)
Outlook does not natively export individual emails as EML in bulk. Options:
- Drag-and-drop: drag emails from Outlook to a folder in Windows Explorer — Outlook saves them as .msg files, not .eml
- Import/Export Wizard: File → Open & Export → Import/Export → Export to a File → Comma Separated Values (for contacts/calendar) or to another Outlook data file (another PST)
- Free tools: MailStore Home, PSTwalker, or Aid4Mail can export PST to EML format
Export to PDF
Individual emails can be printed to PDF (Ctrl+P → Microsoft Print to PDF). For bulk export to PDF:
- MailStore Home (free for personal use): converts PST contents to searchable PDF
- Third-party tools like PST to PDF Converter (paid)
Migrating PST to Gmail, Office 365, or Thunderbird
PST → Gmail: use Google Workspace Migration for Microsoft Outlook (GWMMO) — free tool from Google PST → Thunderbird: Thunderbird with ImportExportTools NG can import PST files directly PST → Office 365 (new tenant): use the Office 365 Import Service (requires tenant admin access)
Opening PST Without Outlook
Several approaches work without a Microsoft Office subscription:
- Thunderbird + ImportExportTools NG plugin: import PST/OST files directly (some PST files require the companion libpff integration)
- LibPST / readpst (Linux, also available for Windows):
readpst -o output_dir/ mailbox.pst— extracts to mbox format - MFCMAPI (Microsoft's free developer tool): low-level access to PST/OST contents, including raw message properties
- Online converters: upload PST and download EML, PDF, or other formats (use only for non-sensitive data)
PST in Legal and Compliance Contexts
PST files are the default output format for e-discovery requests in legal proceedings. Organizations routinely receive legal holds requiring preservation of PST files for litigation. Key considerations:
- Metadata preservation: MSG messages inside PST preserve full Internet headers, including received timestamps and routing information — essential for forensic authenticity
- Chain of custody: PST files should be hashed (SHA-256) before and after review to demonstrate no modification
- E-discovery tools: Relativity, Nuix, and Veritas eDiscovery Platform all ingest PST/OST natively
- Microsoft's recommendation: move from local PST files to Exchange Online / Microsoft 365 for better compliance, audit logging, and litigation hold capabilities
PST files are like a filing cabinet living on your hard drive: capacious, reliable when healthy, but a single point of failure if the drive dies. Regular backups of your PST files (along with the emails in them) are one of the most underrated personal data management practices.
Related conversions
Frequent conversions across the catalogue: