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OST to MBOX Converter

Combine Outlook OST files into one MBOX mailbox for import into Thunderbird, Apple Mail or other clients.

Your files never leave your device

How to convert OST to MBOX

  1. 1

    Choose your OST file

    Click the upload area or drag files onto it. Nothing is uploaded — files are opened locally.

  2. 2

    Start the conversion

    Press the convert button. Your browser converts the files in seconds.

  3. 3

    Download the result

    Save each file individually or everything at once as a ZIP.

Merge many individual messages from Outlook’s offline Exchange and Microsoft 365 mailboxes into a single MBOX file — the easiest way to import a batch of emails into Thunderbird or Apple Mail, or to archive them as one file.

Read the full guide with screenshots

What is OST?

OST is Outlook’s offline copy of an Exchange or Microsoft 365 mailbox. It has the same structure as a PST but is normally tied to one account — converting it rescues mail from old or orphaned profiles.

What is MBOX?

MBOX stores a whole mailbox — thousands of emails — in one file. Google Takeout, Thunderbird and Apple Mail exports all use it.

Is it private?

Yes. Unlike upload-based converters, ZapMorph processes files inside your browser tab, so the file stays on your device. It is never transmitted, stored or logged — close the tab and it’s gone.

Frequently asked questions

Is the ost to mbox converter free?

Yes. Convert 1 file a day without an account, or 5 files a day with a free account. Pro removes all daily limits.

Are my files uploaded to a server?

No. The conversion runs entirely in your browser, so your files never leave your device. We only count how many files you convert — never their names or contents.

What is the maximum file size?

Up to 25 MB per file without an account, 50 MB with a free account, and 1 GB on Pro. Very large files also depend on your device’s memory.

Can I convert multiple OST files at once?

OST to MBOX Converter works on one file at a time and can produce many output files, which you can download together as a ZIP.

Does it work on Mac, Windows, iPhone and Android?

Yes. It works in any modern browser — Chrome, Edge, Firefox and Safari — on desktop and mobile, with nothing to install.

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