PST
OSTPersonal Storage Table — Microsoft Outlook's container file for an entire mailbox, including folders, messages, contacts, and calendar items. Also known as OST when used for offline sync with Exchange or Microsoft 365.
A PST (Personal Storage Table) file is a database-like container created and used by Microsoft Outlook on Windows to store email folders, messages, contacts, tasks, and calendar events in a single file. OST (Offline Storage Table) is a closely related format used when Outlook synchronizes with an Exchange or Microsoft 365 server, keeping a local offline copy of the mailbox.
PST files can grow very large — Microsoft recommends keeping them under 50 GB for performance reasons, though they can technically hold more. Because PST is a proprietary binary format, extracting data from it without Outlook requires specialized tools. Corruption is also a known risk with very large PST files, which is why many administrators encourage users to archive to more portable formats.
Users who want to migrate away from Outlook or Windows typically export their PST to MBOX or EML first, using tools such as the free libpst/readpst utility or third-party converters. Once in MBOX format, the archived mail can be opened in a wider range of applications.
Related terms
A plain-text file format that stores multiple email messages concatenated together, each beginning with a "From " separator line. It is the format Google Takeout produces when you export your Gmail archive.
Microsoft Outlook's proprietary binary format for a single email message, storing headers, body, and attachments in a Compound Document Structure. Unlike EML, MSG is not a plain-text standard.