You found an old .pst file from a previous job or computer, or a backup of your mailbox, and Outlook isn't installed. A .pst file is an Outlook data file: a whole mailbox with folders, emails and attachments. Here are your options.
| Option | Cost | File leaves your PC? | Best for |
|---|---|---|---|
| 1. Classic Outlook (if you have it) | Microsoft 365 | No | Moving mail back into a mailbox |
| 2. A desktop PST viewer | Free to view | No | Reading or exporting old mail |
| 3. Thunderbird import | Free | No | Usually needs Outlook on the same PC |
| 4. Online PST converters | Varies | Yes, uploaded | Non-sensitive mailboxes only |
If you have classic Outlook for Windows, use File → Open & Export → Open Outlook Data File. The mailbox appears as a separate folder list. This is the right choice when you want to move the old mail back into your current mailbox.
A viewer opens the .pst file directly and shows the folder tree, the email list and each message with its attachments – no Outlook, no email account. Because it runs on your PC, old mail is never uploaded.
Our own MSG, EML & PST Viewer – Email File Reader for Windows is free for opening mailboxes, reading emails and saving attachments one at a time. Large mailboxes open quickly because emails load one folder at a time. A one-time Pro upgrade adds PDF and EML export for single emails, folders or the whole mailbox, and CSV email lists.
Mozilla Thunderbird can import Outlook mail, but its import usually relies on Outlook being installed on the same PC. Without Outlook it often can't read a .pst file directly.
Some websites convert .pst files to other formats. You have to upload your whole mailbox to someone else's server, and mailboxes are often several gigabytes. Avoid them for personal or work mail.
An .ost file is Outlook's offline copy of a mailbox that lives on a server (Microsoft 365, Exchange, Outlook.com). If the account still works, the easiest way to see the mail is to sign in to that account. If the account is gone, a viewer can often still read the .ost file. However, .ost files encrypted by a company security policy can't be opened by viewers.
A .msg file is one saved email. A .pst file is a whole mailbox with many folders and emails.
Yes, in a viewer that reads one folder at a time instead of loading everything at once. Multi-gigabyte files are common.
A viewer only reads the file. Still, keep a backup copy of important mailboxes before working with them.
Close Outlook or any other program that has the mailbox open, then try again.