Convert an OST File to MBOX.
An .ost is Outlook's offline cache, and it is the file people are left holding when a profile is deleted or a mailbox is closed. Open it directly on your Mac, iPhone, or iPad — with no Outlook installed, no account, and no Exchange server — see exactly what is inside, then convert the mail to MBOX for Apple Mail, Thunderbird, and anything else that reads it.
100% on-device | .ost → MBOX · PDF · HTML | Orphaned OST supported | No Outlook · No Exchange | Nothing uploaded
An OST file is not an export. It is the offline cache Outlook keeps so it can show your mail without a connection, and it is tied to the profile and account that created it. That is why it becomes a problem: when the profile is removed, the laptop is replaced, or the mailbox is closed, the .ost is still sitting on disk with everything in it — and nothing will open it. Outlook itself will refuse, because the account it belonged to is gone.
OST to MBOX Converter reads the file directly. It does not need Outlook installed, it does not need the original account, and it does not need to reach an Exchange server. An orphaned OST left behind by a profile that no longer exists opens the same as any other.
It is an opener and a converter rather than a mail client. Pick the .ost and you get a clear breakdown of what is inside — how many folders, messages, and attachments — so you can confirm it is the right mailbox before exporting anything. Then convert it to MBOX, which is what Apple Mail, Thunderbird, and most other mail apps actually import; to a clean printable PDF; or to a browsable HTML archive that opens in any web browser.
Large mailboxes are handled the way they need to be. You can keep the original folder structure or flatten it, skip duplicate messages, and split the output into parts so it fits the import size limits of whatever you are moving to. Every step runs on your device, so a mailbox full of company mail is never uploaded anywhere.
What an OST can become.
The same file, exported into whichever form the thing you are migrating to will actually accept.
OST → MBOX
The format Apple Mail, Thunderbird, and most non-Outlook mail apps import.
OST → PDF
A clean, printable copy of the mailbox for archiving or disclosure.
OST → HTML
A browsable archive that opens in any web browser, no mail app required.
Orphaned OST
A cache left behind by a deleted profile opens with no account and no server.
How to convert an OST file to MBOX.
Add your .ost file
Open it from the Files app, a download, AirDrop, or the share sheet. It is read on your device — no Outlook, no Exchange server, and nothing uploaded.
Check what is inside
The app reports the folder tree and the number of messages and attachments, so you can confirm this is the mailbox you meant before exporting it.
Choose MBOX and your options
Keep or flatten the folder structure, skip duplicate messages, and split a large mailbox into parts that fit your destination's import limit.
Export and import
Save the MBOX to Files or send it straight on, then import it into Apple Mail, Thunderbird, or whichever app you are moving to.
Both Outlook data files, same conversion.
Outlook's offline cache — including an orphaned file left behind by a profile or account that no longer exists.
Outlook's export and archive format, converted the same way if that is the file you were given instead.
The output: a standard mailbox file ready to import into Apple Mail, Thunderbird, and other mail applications.
Features built for getting mail out of an OST.
No Outlook, No Exchange
The file is parsed directly. Nothing needs to be installed, signed into, or reachable on a server for it to open.
Orphaned OST Support
A cache whose profile was deleted still opens in full — the usual reason an OST becomes unreadable everywhere else.
See Before You Export
A breakdown of folders, message counts, and attachments up front, so you convert the right mailbox rather than guessing.
Keep or Flatten Folders
Preserve the original folder tree in the MBOX, or flatten everything into one mailbox if that suits the destination better.
Duplicate Skipping
Repeated messages — the usual result of a cache that resynced more than once — can be dropped from the export.
Split Large Mailboxes
Break the output into parts so an oversized mailbox still fits the import size limit of the app receiving it.
PDF and HTML Output
Export a printable PDF or a browsable HTML archive when the goal is to keep a record rather than migrate live mail.
Runs on iPhone, iPad, and Mac
The conversion is native on all three, so an OST can be dealt with from whichever device it reached you on.
Parsing, conversion, and export all run locally. A mailbox full of company mail is never uploaded to a server or a conversion service.
No sign-in, no licence check, and no connection to the account the OST came from — which is usually the account you no longer have.
The .ost is read, never rewritten. The export is written separately, so the cache you started with stays exactly as it was.
- Recovering mail from an OST left behind after a work profile or account was closed.
- Moving an Outlook mailbox to Apple Mail or Thunderbird, both of which import MBOX rather than OST.
- Opening an orphaned OST when Outlook refuses because the original profile no longer exists.
- Converting a mailbox on a Mac where Outlook was never installed in the first place.
- Splitting an oversized mailbox into parts that fit another application's import limit.
- Producing a printable PDF or browsable HTML copy of a mailbox for records or disclosure.
Download it and open your first .ost file straight away. There is no account to create, and nothing about your files leaves the device.
Converting OST to MBOX.
What is an OST file?
An .ost is Outlook's offline cache. Outlook keeps a local copy of a mailbox in it so mail is readable without a connection. Unlike a .pst, it is bound to the profile and account that created it, which is why it stops opening when that profile is gone.
Can I convert an OST to MBOX without Outlook?
Yes. The app parses the .ost directly on your device. Outlook does not need to be installed, and there is no Exchange server involved at any point.
My OST is orphaned — the account no longer exists. Will it still open?
Yes. An orphaned OST left behind by a deleted profile opens in full. This is the case Outlook itself usually refuses, and it is the main reason people need a separate converter.
Why MBOX and not PST?
MBOX is what Apple Mail, Thunderbird, and most non-Outlook mail apps import. If you are moving away from Outlook, converting to PST first is an extra step you do not need — though the app can produce a PST as well if Outlook is the destination.
Is my mail uploaded anywhere?
No. Online OST converters require you to upload the entire mailbox to a third-party server. Everything here runs on your iPhone, iPad, or Mac, and nothing leaves the device.
Will the folder structure survive?
Yes, if you want it to. You can keep the original folder tree in the MBOX output, or flatten everything into a single mailbox when the destination handles that better.
My mailbox is very large. Will it fit?
You can split the output into parts sized for the import limit of whatever you are moving to, and skip duplicate messages, which usually removes a large share of a resynced cache.
Do attachments come across?
Yes. Attachments are carried into the MBOX with their messages, and the app reports the attachment count up front so you can confirm before exporting.
Can it convert a PST too?
Yes. The same app opens .pst files and converts them the same way — see the PST to MBOX Converter page if that is the file you have.
What people open next.
Every Rush tool runs entirely on your device. These are the ones that come up in the same job as this one.
- OST to PST Converter.ost → .pstIf the mail is going back into Outlook rather than out of it.
- PST to MBOX Converter.pst → .mboxThe same conversion for a .pst — an export file rather than an offline cache.
- MBOX Viewer.mboxOpen the MBOX afterwards to check the folders and attachments survived.
- PST Viewer.pst · .ostRead the mailbox first if you only need to find one message, not migrate.
Get the mail out of your OST.
Open an .ost with no Outlook, no account, and no Exchange server, then convert it to MBOX, PDF, or HTML — on iPhone, iPad, and Mac, 100% offline.
OST to MBOX Converter is an independent utility from Rush Tools. Outlook, Exchange, Apple Mail, and Thunderbird are named only to describe file compatibility and migration targets.