Open a PST or OST file without Outlook

What a .pst file is

A .pst (“Personal Storage Table”) file is where Outlook for Windows keeps email, calendar, contacts and tasks outside a mail server. You end up with one when Outlook archives old mail (archive.pst), when someone exports a mailbox (File → Open & Export → Import/Export), when an account uses POP, or when an IT department or a court production hands over a mailbox as a file.

Microsoft documents the format publicly ([MS-PST]). Outlook 2003 and later write the Unicode format, which allows files of tens of gigabytes (Outlook 2010 and later stop at 50 GB by default); Outlook 97–2002 wrote an older format limited to 2 GB.

What an .ost file is, and why Outlook refuses it

An .ost (“Offline Storage Table”) file is Outlook’s local copy of a mailbox that lives on a server: Exchange, Microsoft 365 or Outlook.com. Outlook builds it so you can read mail offline, and ties it to the account and the computer that created it.

Outlook has no “open .ost” command. When the account is gone (a former employee, a closed Microsoft 365 subscription, an old PC), the .ost is “orphaned”: Outlook will not open it, but the messages are still inside. If the account still exists, the simplest way to the mail is to sign in to it, because everything in the .ost is also on the server.

Why it won’t open

• No Outlook for Windows. Only the classic Outlook for Windows desktop app opens a .pst (File → Open & Export → Open Outlook Data File). Outlook for Mac can import one but not open it, and Outlook on the web, mobile Outlook, Apple Mail, Gmail and Thunderbird cannot read it at all.
• It is an .ost. Outlook only uses an .ost with the account that created it.
• The file is damaged or incomplete. A .pst copied while Outlook had it open, or cut short, may fail in Outlook. We show what can still be read; Outlook’s own Inbox Repair Tool (scanpst.exe) can sometimes fix the file.
• It has a password. Outlook can protect a .pst with a password. We do not open such files.

What this viewer does

• Opens .pst files from Outlook 2003 and later and .ost files, including the compressed .ost files of Outlook 2013 and later. The older .pst format of Outlook 97–2002 is supported but less tested.
• Shows the folder tree with message counts, and lists each folder newest first, 50 messages at a time.
• Searches the newest 50,000 messages of a folder by subject, sender name and the names in the To line (not the message text).
• Shows a message with its sender, recipients, date and text. HTML emails are shown in a locked-down frame: no scripts run and no remote images load, so nobody learns that you opened the message.
• Bodies stored only as Rich Text (RTF) are converted for reading.
• Saves attachments as files, attached emails as .eml, and any message as an .eml file with its attachments.

What it does not do

• It does not change, repair or convert your file. It only reads it.
• It does not export whole folders at once, convert .ost to .pst, or print to PDF.
• Calendar items, contacts and tasks are listed as items (with subject and text), not shown as a calendar or address book.
• It does not recover deleted messages that Outlook has already purged.
• Password-protected .pst files and files that use Outlook 2003–2007’s optional “high encryption” are not opened.

Why the file is not uploaded

Mailboxes hold private and business correspondence, and they are big. Uploading a 10 GB file would take hours and put it on someone else’s server. Here the browser reads the file from your disk on demand: the folder list, then one page of a message list, then one message. The page’s security rules stop it from sending data to other sites, and neither the file nor its name is sent to our own domain.