What is the PST file format?
A .pst file contains Microsoft Outlook, Messaging, or Exchange Client user personal data. .pst files use the Personal Storage Table format - a proprietary binary database developed by Microsoft that holds an entire mailbox in one portable container.
The current version of the format uses Unicode encoding and allows storing up to 50 GB of data in a single .pst file. Older ANSI-encoded .pst files, used by Outlook up to version 2002, were limited to 2 GB. Every .pst file starts with the 4-byte file signature !BDN (hex 21 42 44 4E) at offset 0; two bytes at offset 8 identify whether the file uses the legacy ANSI layout or the modern Unicode layout.
A .pst file can store:
- Messages
- Contacts
- Scheduled meetings
- Tasks
- Notes
- Calendar entries
.pst files offer some form of encryption and password protection, but the built-in protection is relatively weak - text content inside a .pst can often be read without the password. Microsoft allows users to set one of the following levels of .pst data protection:
- None - data is not encrypted
- Compressible - data is encoded using a simple cipher keyed on the password
- High/Better - a more advanced protection method that still relies on a password
The open-source libpst library and its readpst command-line tool can open both encrypted and unencrypted .pst files on Linux and macOS, and convert them to formats such as .mbox or individual .eml messages. This makes libpst a practical solution for migrating Outlook data to other mail clients, as such usage is not supported by Microsoft itself.
Data stored in .pst files - just like data in .ost files - is stored using fixed-length blocks. When a message is added, the program checks for available free blocks of the right size; if they exist, the data goes into them, otherwise the file expands. When a message is deleted, its block pointer is simply marked as free, which makes recovery of deleted messages possible as long as the pointer has not been overwritten. The flip side is that .pst files can grow quite large even when storing a relatively small amount of live data. The solution is to use the Compact Now feature available in Outlook in order to reclaim disk space.
Microsoft's on-disk format specification (MS-PST) is publicly documented, enabling a broad ecosystem of third-party viewers and converters. The registered MIME type for .pst files is application/vnd.ms-outlook.
Security & safety
RISK: MEDIUMA PST file is data, not an executable, so the file itself won't run on its own - but it can contain hundreds of emails with the usual email threats: malicious attachments, phishing links and tracking pixels that fire when a message is opened in a live client. A PST may also hold highly sensitive personal/business data (it's a whole mailbox), so treat exports as confidential and consider Outlook's PST password/encryption (note: the legacy password is weak and easily stripped - don't rely on it for real security). When choosing a "free PST viewer", stick to reputable vendors: searches for PST tools surface adware-bundled installers and fake "recovery" products, so download from the official site and scan installers.
Format details
in a nutshellPrograms that open PST files
Windows4 apps
Microsoft Outlook
Paid Native opener: File > Open & Export > Open Outlook Data File, pick the .pst; it appears as a folder set in the left pane. 
Aid4Mail
Paid Reads PST directly (no Outlook needed) to view/convert/export messages - useful for migration and e-discovery. 
Outlook ScanPST (Inbox Repair Tool)
Built-in Ships with Outlook (ScanPST.exe). Run it against a damaged .pst to repair corruption before opening - the first thing to try when a PST won't load.
Free PST viewers (e.g. Kernel/SysTools PST Viewer)
Free Standalone read-only viewers open a .pst without Outlook to browse and search messages. Use a reputable vendor; avoid bundled-adware 'viewers'. 
macOS2 apps
Microsoft Outlook for Mac
Paid Import the .pst: File > Import (legacy/new Outlook). Mac Outlook stores mail differently (no live PST), so it imports rather than mounts the file.
Aid4Mail / dedicated converters
Paid On macOS, convert the PST to MBOX/EML to read it in Apple Mail or Thunderbird (Mac Outlook can't open a PST in place). 
Linux1 app
readpst (libpst)
Open-source Command line: 'readpst -o outdir file.pst' extracts the mailbox to MBOX/EML for reading in Thunderbird/Evolution. The standard free Linux route. Technical details
deep spec| File signature | 21 42 44 4E ("!BDN") at byte offset 0 |
| MIME type | application/vnd.ms-outlook |
| Character encoding | Unicode (Outlook 2003 and later); ANSI (Outlook 97-2002) |
| Max file size - Unicode | 50 GB default; adjustable via Windows Registry in Outlook 2010 and later |
| Max file size - ANSI | 2 GB hard limit |
| Internal storage model | Fixed-length block allocation; free blocks are reused on insert and marked free on delete |
| Encryption levels | None / Compressible (XOR substitution cipher) / High-Better (stronger password-keyed cipher) |
| Version identifier | 2-byte field at offset 8: 0x0E or 0x0F = ANSI; 0x17 or 0x23 = Unicode |
| Native application | Microsoft Outlook for Windows and macOS |
| Inbox repair tool | ScanPST.exe (Inbox Repair Tool), bundled with Microsoft Outlook installations |
| Linux and macOS access | libpst / readpst open-source library; Aid4Mail and other third-party converters |
| Related format | .ost - shares the same on-disk structure but is account-bound (Exchange or IMAP offline cache) |
| Stored data types | Email messages, contacts, calendar events, tasks, notes, journal entries |
| Deleted item recovery | Possible until the freed block pointer is overwritten by new data |
| Disk space reclamation | Outlook's Compact Now feature compresses unused block space without data loss |
| Released | 1997 (with Microsoft Outlook 97); modern Unicode PST format since Outlook 2003 |
| Open standard | Yes · royalty-free |
| Specification | learn.microsoft.com |
PST conversions
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