Glossary
What Is a PST File?
A PST file — Personal Storage Table, shown in Outlook as an "Outlook Data File" — is Microsoft Outlook's mailbox container. One proprietary file that can hold an entire account's mail, calendar, contacts, and tasks. Outlook creates one when you export a mailbox or set up a local archive.
PST, OST, OLM — Outlook's three containers
- PST is the portable one: you create it with an export, and you can move it, copy it, and import it into another Outlook. How to export one, step by step.
- OST is an offline cache of a server mailbox (Exchange, Microsoft 365, IMAP). Outlook manages it automatically, it's tied to that account, and it is not a backup you can restore from.
- OLM is Outlook for Mac's own export format — and Windows Outlook can't read it without conversion.
Size limits and corruption
There are two generations. The old ANSI PST (before Outlook 2003) had a hard 2 GB ceiling and was notorious for corrupting near it. The modern Unicode PST is capped at 50 GB by default (a registry setting can raise it). Corruption is still common enough that Outlook ships a dedicated repair tool, scanpst.exe — one reason a single giant PST is a fragile place to keep years of email.
Can you open a PST without Outlook?
Not natively — Apple Mail, Thunderbird, and webmail can't read PST, and the redesigned "new Outlook" app's PST support is still limited. Classic desktop Outlook is the reliable way in. That lock-in is the main argument for keeping long-term archives as per-message .eml files instead: the open RFC 822 format that any mail client opens, one message per file. And if the mailbox in question is a work account, the rules change again — see does Microsoft 365 back up email?
Is a PST a good backup format?
It works — official, free, and complete for the moment you make it. But it's a manual snapshot in a proprietary single-file container: stale immediately, awkward to restore one message from, prone to corruption at size, and a poor fit for cloud storage (OneDrive won't reliably sync a PST that Outlook holds open). Our Outlook backup guide compares the PST route against archiving each message automatically as .eml.
FAQ
What's the difference between a PST and an OST file?
A PST is a portable export you create and control — movable, copyable, importable elsewhere. An OST is Outlook's automatic offline cache of a server mailbox: tied to that account, rebuilt on demand, and not usable as a backup.
How big can a PST file be?
Modern Unicode PST files are capped at 50 GB by default, and administrators can raise that through the registry. The pre-2003 ANSI format was capped at 2 GB and often corrupted as it approached the limit.
How do I open a PST file without Outlook?
There's no built-in way — the format is proprietary, and other mail clients don't read it. The practical routes are importing it into any classic desktop Outlook, or converting the mail out to open formats. If you're choosing an archive format going forward, one .eml per message avoids the problem entirely.
Where is my PST file stored?
Wherever you saved it when you exported. Outlook's own data files live under C:\Users\<you>\AppData\Local\Microsoft\Outlook — but note that the .ost files there are server caches, not backups.