.pst.ost
PST and OST files: what they are and how to open them
A PST is an Outlook data file full of mail folders, and an OST is Outlook’s offline copy of a mailbox. Read either one offline on Android or Windows in Universal File Opener, without Outlook or an account.
Check your PST and OST file here
Nothing is uploaded. This page names the type and shows its details; to view it, use an app below.
What is a PST and OST file?
A PST, short for Personal Storage Table, is the file Outlook uses for exports, archives and some mail accounts. You meet one as an old mail archive, an export from a previous job or a backup copied off a retired computer.
An OST is Outlook’s offline cache of an Exchange, Microsoft 365 or Outlook.com mailbox. Outlook ties it to the account that made it, so a copy moved to another computer usually will not open in Outlook. Universal File Opener reads what is saved in the file, so how much you see depends on how much Outlook cached.
What is inside
A PST starts with the four bytes 21 42 44 4E, which spell BDN after an exclamation mark. Below the header sit a database of nodes and blocks and, on top of it, the folder tree, messages and attachments. Old ANSI files (versions 14 and 15) top out at 2 GB; Unicode files (version 23 and later) can be far larger.
Block data can be scrambled with a simple permute or cyclic scheme. That hides the text from a quick search but is not a password, and Universal File Opener decodes both. A file protected with a password or other encryption is refused.
archive.pstmagic number, version 23├─ headerversion, encoding and root pointers├─ node and block treesthe database layer├─ foldersInbox, Sent Items, Archive …└─ messagesheaders, body (text, HTML or RTF) and attachmentsHow to open a PST and OST file
Windows
- Classic OutlookPaid, part of Microsoft 365 or Outlook 2024. Choose File, Open & Export, Open Outlook Data File to add a PST beside your own mailbox.
- Universal File Opener for WindowsOursReads the folders and messages offline, with attachments, and needs no Outlook or mail account. Read only.
Mac
- Outlook for MacImports a PST into its own mail store, so keep the original until the import has finished.
- readpstFree command-line tool from the libpst project. Turns a PST into MBOX files or one EML file per message.
Android
- Universal File OpenerOursReads the folder tree, messages and attachments offline, with no mail account. Free to open and read only.
In a browser
- Outlook on the webCannot open a PST file. Open it in classic Outlook for Windows, import it into Outlook for Mac, or read it in the Android or Windows app.
Won’t open?
Outlook says the file is not a personal folders file
It may be an OST, which opens only in the Outlook profile that made it, or a damaged or renamed file. Open it in Universal File Opener to see whether the folders can still be read.
The app refuses the file
A PST with a password or other encryption is refused. Remove the protection in Outlook or ask the owner for an export. Damaged files that cannot be read are refused too.
Only some messages are listed
The list shows the first 1,000 messages in each folder and 10,000 overall. Use Outlook to export a smaller PST if you need the rest.
Calendar and contacts are missing
The apps list mail folders and messages. Calendar items, contacts and tasks have no view of their own. Save the calendar from Outlook as an ICS file, and export contacts as a CSV file.
An OST has less mail than the mailbox
An OST holds only what Outlook cached for offline use. The rest stays on the server, so sign in to the account to see it.
Convert a PST and OST file
- To MBOX or EMLOn a Mac or Linux with libpst installed, create a folder named out, then run readpst -r -o out archive.pst for MBOX folders, or add -e for one EML file per message. Keep the original PST.
- To another Outlook mailboxIn Outlook choose File, Open & Export, Import/Export, then Import from another program or file and pick Outlook Data File.
PST and OST files in our apps
Universal File Opener reads Outlook PST and OST files offline on Android and Windows, without Outlook or a mail account. Browse the folder tree, open messages with their bodies and attachments, and search. Files are read only.
All Android tools and limits for PST and OST files
- Shows the folder tree and lists messages, up to 1,000 per folder and 10,000 in all. Messages open with plain, HTML or compressed RTF bodies and their attachments.
Limits
- Password-protected and encrypted files are refused. Calendar items, contacts and tasks have no view of their own.
- An OST holds only what Outlook cached for offline use. Saved search folders show counts only.
- The file stays read only: no synchronization and no editing.