Folders
Messages
Nothing in this folder matches the filter.
Pick an item on the left and it opens in this pane.
Read the mail stuck inside a disconnected Exchange or Microsoft 365 cache without Outlook, without a server login and without anything leaving your machine.
Or pick one from the machine you are on. The old mailbox opens in the tab and is read where it is, with nothing sent out.
An OST file, short for Offline Storage Table, is the copy Outlook keeps on your drive in Cached Exchange Mode. It is tied to the profile that made it and scrambled with a key from that account. Once the mailbox is gone Outlook treats the file as out of date and shows nothing, though the mail is still there. This page skips that check and rebuilds the folders.
Outlook wants the original account. The reader asks for nothing, so a deleted profile's mailbox opens on a new machine.
Nothing to set up on Windows, Mac, Linux or a Chromebook. Chrome, Edge, Firefox or Safari turns it into readable mail.
Save a message as .eml or .txt, pull one attachment, zip an item's files or send the list to a spreadsheet.
In brief: any machine with a current browser can read it. Windows, macOS and Linux desktops, Chromebooks, iPhones, iPads and Android phones all work, with no Exchange or Outlook needed.
Outlook for Mac will not open a cache from another account. Open it in Safari or Chrome and the mailbox appears with its folder tree.
A Chromebook has no Exchange client and no cache reader. Load the .ost from Files or Drive and its mail appears in the tab.
Firefox or a Chromium browser on any Linux system reads the file, which helps on a computer with no Microsoft mail app.
Move the cache into the Files app on iPhone, or the Downloads folder on Android, then open it here. Smaller caches suit a phone.
Edge or Chrome opens a cache with no profile to rebuild. For steady recovery on a PC, the free Univik OST Viewer for Windows is the desktop tool.
A work or borrowed computer blocks installers, but a browser tab needs no admin rights.
The reader goes through the offline storage format one record at a time, the way Outlook once did. Everything below is turned back into readable mail on your own computer.
The older Unicode caches from Outlook 2007 and 2010, plus the 4 KB page layout that Outlook 2013 through 2024 and Microsoft 365 produce. Exchange Server, Exchange Online, Outlook.com and IMAP accounts all make one. The type shows in the summary once loaded.
Outlook hides the cache with its compressible method, and older files with the high one. Both are undone as the page reads.
Newer caches squeeze their contents into compressed blocks, and older messages use Outlook's own rich text packing. The page opens both into readable mail.
One tab shows the designed message with its images, the next the wording alone. Rich text is rebuilt as a formatted page, not skipped.
Save documents, spreadsheets, pictures and archives, including an email attached inside another. Large files are stored in several pieces and are put back together first.
Titles and bodies in Cyrillic, Greek, Thai, Hebrew, Arabic or the East Asian scripts. Shift_JIS, GBK, Big5, EUC-KR, KOI8 and the ISO and Windows code pages all decode correctly.
Trash items are shown in red so they are never mistaken for current mail. Ones removed by Empty Folder are looked for in the pages Outlook left behind and collected under Recovered items, while that space is unused.
Healthy caches open, and so do the rest. One that never finished syncing, or that Outlook flags as corrupted or inaccessible, still returns most of its mail.
A cached mailbox holds more than email. Meetings, contacts, tasks, notes and journal entries each carry a label. One kind can be picked out of a full folder.
Drag the .ost into the box above or press Choose. Open several at once and switch between them, or use Select a folder for a directory.
The folder tree loads first, then the list. Search by sender, subject, recipient or folder, sort by date, then open a message with headers, metadata and files kept whole.
The Download menu gives four choices: a message as .eml or .txt, one file alone, all files zipped together or the list as a spreadsheet.
A cache loses its mailbox in a few ways, and each leaves the mail readable here after Outlook blocks it.
The employer switched off the Exchange account, but the cache was already on the laptop. Outlook will not open it, the mail opens here.
When a subscription ends the sync stops, yet the local copy stays on disk. This page reads it as it is.
A move to new systems or the cloud leaves the old Exchange server gone. Caches from it have no server to reach, which is fine here.
A new Windows install or a fresh Outlook profile no longer recognises the cache. The reader needs no profile, so it does not matter.
A cache from an old laptop, a disk image or a relative's computer has no account to attach to. It still opens.
A cache handed over for eDiscovery or digital forensics has to be read without touching a server. Opening it here changes nothing.
Both tools are Univik's. This page reads a stranded cache on any computer. When the mail must become a portable file, the Univik OST Converter saves it as PST and other formats. To read on a Windows PC instead, the free Univik OST Viewer for Windows does the same job. Background on the two file types is in the OST vs PST guide.
| What you need | Online OST Viewer | Univik OST Converter |
|---|---|---|
| Setup | None, the page does the work | Windows installer |
| Runs on | Any device with a browser, phones included | Windows 7 to 11 |
| Where the cache goes | Nowhere, it stays on your device | Nowhere, the PC handles it locally |
| Opens orphaned caches | Yes, no account or server | Yes, no account or server |
| Reading the mailbox | Folder tree, styled view, text, headers | Preview ahead of conversion |
| Keeping one message | .eml or .txt right away | Any chosen output |
| Bulk export | One item at a time | Whole folders in a single run |
| What it writes out | One message to EML or TXT, saved files and a CSV of the list | A portable PST, plus PDF, MSG, MBOX, HTML and more |
| Turn a cache into a portable PST | Not its job | Its whole purpose |
| Price | Free | Trial first, licence after |
Plain answers about reading a disconnected Exchange or Microsoft 365 cache in the browser, with no sign-in, no server behind it and no Outlook on the machine.
It does not. The tab reads the cache a piece at a time on your own computer, and no part of it is sent anywhere. Turn off the internet once the page has loaded and the mailbox still opens. Images stored online stay blocked until you ask for them.
Because it lost the mailbox it belonged to. Outlook binds each cache to one profile, so once the job ends or the server retires, it calls the file orphaned and blocks it. The mail inside is fine, and this reader opens it without the account.
If the cache is on a machine you hold, the reader opens it though the Exchange account is gone. One thing to check first: many employers treat work mail as company property, so be sure you are allowed to read it.
Yes. Caches from Outlook 2007 and 2010 use the earlier Unicode layout, while Outlook 2013 onward, including Microsoft 365, write the 4 KB page layout. Both open here, live subscription or not.
On Windows it sits at C:\Users\<your name>\AppData\Local\Microsoft\Outlook. Paste %localappdata%\Microsoft\Outlook into the File Explorer address bar to go straight there, since AppData is hidden. The file is usually named after the account. An administrator can move it, and a copy from a backup drive opens from wherever you put it.
Usually not. Every cache is scrambled with a key Outlook derives from the account, and the reader reverses it as it reads, so a normal cache opens with no password. A file wrapped by separate encryption stays sealed.
A multi-gigabyte cache is fine on a desktop or laptop, since only the parts in use are read. A phone has less memory, so open large caches on a computer.
Yes. The Attachments tab shows each file with its type and size, previews images and downloads any one alone. The Download menu zips the set.
To read the mail or pull a file, this page is enough and installs nothing. To keep the mailbox or load it into Outlook again, turn it into a portable PST with the Univik OST Converter, which writes the folders, attachments and read and unread marks into a file any Outlook opens.
Often, yes. Mail still in Trash opens like any other folder and is shown in red. Mail removed by Empty Folder is harder: Outlook takes it off the list but leaves the pages behind, so the reader goes through those pages and rebuilds what it finds under Recovered items. On a test cache where four messages were emptied, three came back complete with sender, date, body and headers. The fourth had its space used again, which nothing can undo, so copy the file early.
No cost, nothing sent anywhere, ready in the tab already in front of you.
Choose an .ost file