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PST (Personal Storage Table) is Microsoft Outlook's local storage format for emails, calendar events, contacts, tasks, and notes. If you've ever used Outlook with a POP3 email account or created a local archive, all that data lives in a .pst file — a single, often enormous file that represents your entire communication history.
The format is a structured binary file using a B-tree-based node database (NDB layer), a lists/tables/properties layer (LTP), and a messaging layer that implements the MAPI property schema. Modern PST files (Unicode format, introduced in Outlook 2003) support up to 50 GB, though performance degrades well before that limit. The older ANSI format was capped at 2 GB — and many IT professionals have war stories about PST files hitting that wall and corrupting.
PST files are a love-hate fixture of enterprise IT. They're the standard way to archive Outlook data, migrate between accounts, or back up mailboxes. They're also a compliance nightmare — PST files can be copied, moved, and hidden on local drives, making them impossible to search centrally for legal discovery. Microsoft has been pushing organisations toward Exchange Online and in-place archiving for years, but PST files persist because old habits (and old mailboxes) die hard.