What does the OST to MBOX converter actually do?

It opens an Outlook OST file and saves its mail as MBOX files, the folder-based open format that Thunderbird, Apple Mail and Linux clients use. It works with no Outlook and no live account, so even a left-behind OST file becomes MBOX you can open on any system.

Is Outlook needed to convert OST to MBOX?

No. The OST file is read on its own, so you do not need Outlook or the profile that made it. Outlook has no MBOX support at all, so a separate converter is the only real way to go from OST to MBOX, and it is why dead and orphaned files still convert.

What are the steps to convert OST to MBOX?

Add your OST files, look through the folders, choose MBOX and a save location, then start. The tool writes one MBOX file for each Outlook folder.

Why is there one MBOX file per folder?

That is how MBOX works. One MBOX file holds one folder of messages. So Inbox, Sent Items and every other folder each get their own file, and together they rebuild your original folders when you import them.

Are folders and attachments kept in the MBOX output?

Yes. The folders arrive as a matching set of MBOX files, and each message keeps its attachments, inline images and header fields such as From, To, Cc, Subject and Date just as they were.

Can I batch convert several OST files to MBOX?

Yes. Add a folder of OST files and the tool converts them all to MBOX in one run, keeping each mailbox in its own output folder.

Which email clients open the MBOX files?

Many email clients do: Thunderbird, Apple Mail, Evolution on Linux, Eudora, Postbox, The Bat! and Opera Mail. Gmail can also bring MBOX in with its own import tools.

How do I convert OST to Thunderbird?

Thunderbird reads MBOX, not OST, so there is no direct OST to Thunderbird import. Convert the OST file to MBOX with this tool, then import the MBOX files into Thunderbird with the ImportExportTools NG add-on. Each Outlook folder becomes its own Thunderbird folder, with attachments, dates and read or unread flags kept.

Does it handle Exchange and Microsoft 365 OST files?

Yes. OST files from Exchange, Microsoft 365 and IMAP accounts all convert to MBOX, with the folder layout saved as separate files.

Is my original OST file left untouched?

Yes. The OST file is opened read only and never changed. Everything is copied into new MBOX files, so the original stays exactly as it was for backup or evidence.

Is the OST to MBOX converter free?

The free trial shows the whole mailbox and converts a limited number of items per folder so you can check the result. A license removes the limit for unlimited OST to MBOX conversion.

How is OST to MBOX different from OST to EML or OST to PST?

MBOX puts a whole folder into one open file that Thunderbird, Apple Mail and Linux clients read, which is why it is the format people use to move off Outlook. EML saves each message as its own file. PST keeps the mailbox in one Outlook container. For single files use the Univik OST to EML Converter, for an Outlook container the OST to PST Converter, and to convert existing MBOX files to other formats the Univik MBOX Converter.

OST to MBOX Converter - Convert OST to MBOX Without Outlook

Capabilities

Powerful OST to MBOX Features

Built end to end for accurate, large-scale OST to MBOX exports that lose nothing along the way.

OST Mailbox to MBOX

Convert an Outlook OST file to MBOX, the open format Thunderbird, Apple Mail and Linux clients read. Folder by folder, your mailbox is saved as MBOX.

No Outlook Needed

You do not need Outlook, an Exchange server or a profile. Mail is read straight from the OST file, which is why dead and orphaned files still convert.

Batch Conversion

Point the tool at a folder of OST files and let it run. Every mailbox is converted to MBOX in one go, each kept in its own output folder.

One MBOX File per Folder

Since one MBOX file holds one folder, Inbox, Sent Items and every other folder is saved as its own file, so your folder structure stays intact.

Thunderbird Ready

The MBOX files are written in the version Thunderbird and Apple Mail expect, with correct message breaks, so they import without a repair step.

Attachments and Metadata Kept

Attachments, inline images and every header field stay unchanged, so a converted folder looks just like the original, dates and all.

Pick Folders and Dates

Choose the exact folders you want and set a date range, so a large mailbox gives you just the MBOX files you need.

Exchange, Microsoft 365 and IMAP

OST files from Exchange, Microsoft 365 and IMAP accounts all convert to MBOX, from small archives to very large mailboxes.

Made for Mac and Linux

MBOX is the format that moves mail off Outlook. The files open on Windows, Mac and Linux, so this is the easiest way to reach Apple Mail, Evolution or Thunderbird.

Tricky Files

Difficult OST Files, Handled

The OST files that a manual export cannot handle are exactly the ones this tool is built for.

Orphaned OST files

Mail left behind by a deleted account or profile is read in full and saved as MBOX you can open anywhere.

Damaged OST files

When Outlook fails to open a corrupt OST file, the tool still recovers what it can and saves it to MBOX.

Large mailboxes

Ten, thirty, fifty gigabytes, the tool works folder by folder instead of slowing down or forcing you to split the file.

Encrypted OST files

Standard account-level OST encryption is handled for you, so there is no key to find before conversion.

Any account type

From older Exchange OST files to today's Microsoft 365 and IMAP files, all convert to MBOX.

Old ANSI files

Old ANSI OST files from early Outlook versions convert to MBOX just like modern Unicode ones.

Simple Process

How to Convert OST to MBOX in 4 Steps

No scripts and no Outlook. A few clicks on any Windows PC turn an OST file into MBOX.

1

Install the Tool

Set up Univik OST to MBOX Converter on any Windows PC. You do not need Outlook or an Exchange profile.

2

Load Your OST

Use Add File or Add Folder, or drag files in, to load one OST file or many at once. Orphaned files are fine too.

3

Pick Folders and MBOX

Look through the folders, narrow them with optional date or folder filters, then choose MBOX and where to save the files.

4

Run the Export

Start the conversion. Each Outlook folder is saved as its own MBOX file, ready to import into Thunderbird, Apple Mail or a Linux client.

Common Situations

Common Conversion Scenarios

Convert OST to Thunderbird

Thunderbird is the most common destination. Convert OST to MBOX and each folder imports into Thunderbird as its own local folder.

Migrate Outlook to a Mac

MBOX imports straight into Apple Mail, so OST to MBOX is the simple way to bring old Outlook mail onto a Mac.

Switch to a Linux mail client

Evolution and Thunderbird on Linux both read MBOX, so the format is the easiest way to move from Outlook to Linux.

Server or account shutdown

When a server or Microsoft 365 account is shut down, the OST file is all that is left. MBOX keeps that mail in an open format that no server can lock.

Rescue a left-behind OST file

A copied or left-behind OST file that Outlook will not open still works here, and MBOX makes the rescued mail easy to move.

Open-format archive

Keep mail as plain MBOX files that stay readable without Outlook, a license or a live mailbox.

After Conversion

How to Import Your MBOX Files

After the conversion, each MBOX file goes straight into the client you are moving to.

Import OST into Thunderbird

To import OST into Thunderbird, install the ImportExportTools NG add-on, right-click a local folder and choose Import mbox file, then select the converted MBOX. Each file becomes its own Thunderbird folder.

Import MBOX into Apple Mail

On a Mac, open Apple Mail and choose File then Import Mailboxes, pick Files in mbox format and select the converted MBOX files. The folders appear under Import.

Import MBOX into Evolution or Linux

In Evolution choose File then Import, select Import a single file and pick the MBOX. Most Linux mail clients read MBOX directly without an add-on.

On a Mac or Linux? The converter runs on Windows, but MBOX works on every system. Copy the MBOX files to a Mac for Apple Mail or to Linux for Evolution or Thunderbird, and your Outlook mail reaches the new system without Outlook installed there.

Specifications

System Requirements

Operating System	Windows 11, 10, 8.1, 8, 7 (32-bit and 64-bit)
Processor	1 GHz or faster
RAM	2 GB minimum (4 GB recommended for large OST files)
Disk Space	100 MB for installation, plus space for the output MBOX files
.NET Framework	4.8 or higher
Outlook	Not required for conversion
Supported OST	OST from Outlook 2003 to 2021 and Microsoft 365, including Exchange, IMAP and cached mode
Supported Output	MBOX, one file per folder, opens in Thunderbird, Apple Mail, Evolution and more

The Folder Tree

From OST Folders to MBOX Files

MBOX stores one folder per file, so the main job is turning one OST mailbox into a matching set of folder files.

Inside an OST file, mail sits in folders, Inbox, Sent Items, Deleted Items and any other folders you made over the years. MBOX cannot hold all of those in one place, because one MBOX file holds just one folder of messages. That difference is the main challenge of this conversion.

The converter solves this by going through the OST folders and creating a separate MBOX file for each one it finds, then naming and arranging those files so the structure is clear when you import them. Subfolders keep their path, empty folders are left out and the order you saw in Outlook is the order you get in MBOX. The result is not one giant file but a clean set, one MBOX file per folder, that your new client opens as the same mailbox.

All folders are read

Every folder in the OST file, system and custom, is read before a single message is written.

One file per folder

Each folder becomes its own .mbox file, which is the only way MBOX can hold a mailbox with many folders.

Structure kept

Subfolder paths are kept in the file names, so the client you import into rebuilds the same folders.

Clean output

Empty folders are skipped and duplicates avoided, so you get exactly one MBOX file per real folder.

When to Use It

Why Export OST to MBOX?

Converting OST to MBOX turns a locked Outlook file into an open mailbox format that Thunderbird, Apple Mail and most Linux clients read directly. It is the natural choice for moving off Outlook, especially onto a Mac or Linux, or for keeping a long-term archive in an open format.

To understand the source format, read our guide on what an OST file is, and to learn about the target format see what an MBOX file is. If you already have MBOX files and need to convert them to another format, use the Univik MBOX Converter. To keep everything in one Outlook container instead use the OST to PST Converter, or for single-message files the OST to EML Converter.

One file per folder	Each Outlook folder becomes its own .mbox file, so the folder structure is kept as a clean set of files.
From separator lines	Every message starts with a From line that marks where it begins, just as Thunderbird and Unix mail tools expect.
mboxrd escaping	Any line in the message that begins with From is changed to >From, so messages do not split in the wrong place when the file is imported.
Full RFC 5322 messages	Each message inside the MBOX keeps its original headers and MIME parts, so attachments and inline images survive.
Correct mbox variant	The files match the MBOX version Thunderbird and Apple Mail read, so they import without a repair or rebuild step.
Status headers	Read and unread status is saved with X-Mozilla headers, so Thunderbird shows the right flags after import.
Stable message order	Messages are written in their original order with their original dates, so the folder looks the same as in Outlook.

Univik OST to MBOX Converter

Convert OST to MBOX Without Outlook

Powerful OST to MBOX Features

Everything is Preserved

Difficult OST Files, Handled

Forensic-Grade Integrity

How to Convert OST to MBOX in 4 Steps

Univik Converter vs Manual Methods

Common Conversion Scenarios

Where Your MBOX Files Open

How to Import Your MBOX Files

Convert OST to Thunderbird

System Requirements

What Users Say

From OST Folders to MBOX Files

How Your MBOX File Is Built

Why Export OST to MBOX?

Frequently Asked Questions

More Univik Converters