Free Online OST Viewer
Read a disconnected Outlook cache without the account that made it. No Exchange, no profile, no install.
Built by OnlinePCApps, Outlook formats since 2013
When Outlook Says It Is Not an Outlook File
That error message is where most people start. Here are the three routes that lead to it.
The Account Behind It Is Gone
Exchange was decommissioned, the tenant was closed or the mailbox was deleted. Outlook now answers with an error saying the file is not an Outlook data file.
A Leaver's Laptop Came Back
A deprovisioned account left a cached mailbox behind on a returned laptop, and somebody has to establish what was in there before the disk gets wiped.
It Is Not a Windows Machine
Every tool for this job is a Windows executable, and the file has landed on a Mac, a Linux box or a locked-down laptop.
Read an OST File in 3 Steps
Locate the Offline File
OST files sit under AppData Local Microsoft Outlook. Drag the one you want onto the panel and it opens where it lies.
Open the Folder Tree
The cached hierarchy appears as it stood at the final sync. Mail, contacts and calendar all list together.
Lift Out What Matters
Save an attachment, write an item out as EML or print it. No profile is created and nothing is registered on the machine.
An OST Reader That Is Not a Windows App
Search for a way into a cached mailbox and every answer is an installer for Windows. That is the gap this fills.
On Your Phone
A small cache opens fine. Anything into the gigabytes will exhaust a handset first.
On a Tablet
No tablet has ever shipped anything that reads a cached Outlook mailbox.
On a Locked-Down Laptop
Where policy forbids installing software, the browser is still there and still enough.
On Someone Else's PC
Nothing written to their disk and no profile registered. Shut the tab and it is finished.
The OnlinePCApps OST Viewer is a free browser-resident reader for Outlook offline data files. It rebuilds the cached folder tree from a .ost that no longer has an account behind it, shows mail beside the address book, the diary, tasks and notes, and asks for no Exchange connection, no Outlook profile and no upload. When a cache outgrows the memory a single tab can spare, the desktop build carries it.
What Makes Outlook Write an OST at All
Four account types produce one, and only one of them is the Exchange setup everybody assumes.
Cached Exchange Mode
The classic source. Outlook holds a MAPI profile against the server and mirrors the mailbox locally so it keeps working when the network does not.
Microsoft 365
Exchange Online caches in exactly the same way, and Outlook.com accounts do too. The file is still an OST long after the tenant has gone.
IMAP Accounts
Outlook 2013 onward writes an OST rather than a PST for IMAP too, which surprises people who expected a PST.
Shared and Delegate Mailboxes
Anything a profile has permission to open gets cached beside the primary account.
This OST Viewer vs a Windows Download
Every OST tool on the first page of results is an executable you have to install. That is a fair approach, and it is not free of cost either.
| What differs | This page This page Runs in your browser | A typical desktop OST tool Installs a binary on your machine |
|---|---|---|
| Operating system | Windows, macOS, Linux | Windows only, without exception |
| Installation | None at all | A download and an installer |
| Admin rights | Not needed | Usually needed to install |
| Exchange connection | Seconds | Download, install, then scan |
| Outlook profile | Nothing | Nothing, but you installed their binary |
| What reaches a server | Never created | Never created either |
| Time to first folder | Windows, macOS, Linux | No |
| Removing it afterwards | Close the tab | Uninstall and clear the leftovers |
That covers how the category behaves rather than singling anyone out. Several of those tools are good at what they do.
Your OST File Never Reaches Us
An abandoned cache is rarely your own correspondence. It usually belongs to somebody who has left, and reading it is already a decision that deserves care.
So nothing here collects one. No endpoint accepts a mailbox, no component runs on our side, and no record exists that you opened anything at all. Open the browser network view and drop a file in if you would sooner confirm that than accept it.
→ external requests ....... 0
→ the cached store ....... 0 requests
What an OST File Holds and Why
An OST was built to be a cache rather than an archive, and almost everything awkward about one follows from that single design choice.
| Part | Header | What it holds |
|---|---|---|
| Purpose | offline cache | An OST mirrors a server mailbox so Outlook keeps working with the network down. It was never meant to be a portable archive. |
| Profile binding | entry ID and account | The file records which profile and which account created it. That binding is why Outlook refuses a file it did not make. |
| Underlying layout | same node database | Structurally an OST is built the same way a PST is: a B-tree of nodes beneath a property store. It is not a separate format at all. |
| Sync state | per-item change keys | Every item carries the change key it had at the last sync, which is how Outlook knew what still needed sending. |
| Cached range | sync slider setting | Outlook only caches as far back as the profile told it to. Anything older than that window was never written to disk. |
| Unsent items | local-only nodes | Drafts and queued mail that never reached the server exist only here. When the account goes, they go with it unless the file is read. |
| Deleted items | retained nodes | Items removed from the folder view often survive in the store until the file is compacted, which is why triage tools look here. |
| Encryption | compressible or high | The same optional obfuscation Outlook applies to a PST. It scrambles the bytes on disk and stops no reader that understands it. |
| Item classes | IPM.Note and the rest | IPM.Contact, IPM.Appointment and the rest sit alongside IPM.Note, because this is a mailbox rather than a mail file. |
| Size ceiling | 50 GB by default | Unicode files have a configurable ceiling in the registry. Long-serving cached mailboxes reach it more often than people expect. |
Where the format is documented
Microsoft publishes the layout of its own mailbox files, which is the only reason a reader written by anybody else can walk the same tree. These are the documents this app was built against.
Identifiers absent from MS-OXPROPS, are properties an add-in invented, numbered within one mailbox instead of globally. They resolve as well.
Other Ways to Open an OST File
Four other routes exist. Each works in the right circumstances, and each asks for something the person holding an orphaned cache usually does not have.
Reconnect the Original Profile
Export From a Live Mailbox
Convert It to PST First
Restore From Backup
Check the Header Before Anything
Look under AppData Local then Microsoft Outlook, which is where the file lives, and its first bytes declare whether it is ANSI or Unicode. That single field explains a surprising number of failures.
Look for the Archive Beside It
Look under Documents then Outlook Files, which often holds a separate PST containing everything AutoArchive moved off the server. The cache and the archive together are usually the whole picture.
When an OST File Grows Past the Browser
One tab holds only so much. The desktop build takes the caches that sit above that line.
What the Desktop Build Adds
Resolving the tree means holding the store in memory. For a cache of a few gigabytes that is fine. A mailbox that has been cached for a decade and sits near its 50 GB ceiling is a different proposition, and a phone reaches its limit far sooner than a laptop.
Working through the file in chunks straight off the disk is the desktop approach, and it takes size out of the conversation.
Nothing leaves the machine once it is installed, which is what isolated networks and evidence rooms require.
A long-serving cached mailbox at the 50 GB mark streams from disk rather than sitting in RAM.
Point it at a folder of recovered caches and move between them instead of opening one file at a time.
A whole shelf of recovered caches, queued past what one tab could hold.
Locate an item anywhere in the tree by sender, date or body rather than folder by folder.
Write a whole store out to PST, MBOX, EML or PDF in a single pass.
Questions About Orphaned OST Files
More Outlook & File Apps
The same principle throughout: your device does the work.
Who Built This OST Reader
That OST File Outlook Will Not Open
Hand it over and the hierarchy comes back. No Exchange, no profile, and the disk is untouched.