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Convert OST to MBOX Online


The free online mail converter that transforms EML, MSG, PST, and MBOX files into PDF, HTML, Word, and more — no software required.

CoolUtils online mail converter supports all major email formats as input — EML, MSG, PST, OST, and MBOX — and converts them to PDF, HTML, DOC, TXT, JPEG, and other output formats. Upload the file, select the output format, and download the converted document in seconds. No installation, no registration, completely free.

1) Upload OST file to convert

 

Drop files here, or Click to select
0 %

Allowed file types: pst, ost, eml, msg, mime, smime, p7m, mbox, dbx, vcf, vmbx, opf, asice, cpgz, lzh, zcf

2) Set converting OST to MBOX options

Convert to

 

Options

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3) Get converted file

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Get Total Outlook Converter to convert your Outlook emails safely.

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okConvert Outlook emails from multiple sources to a variety of formats.
okFilter emails by sender, subject, date, keywords, and more.
okUseful data saving options - convert only necessary data from your emails.
okUnique output files formatting options specify how your files will look.
okAffordable Price - pay once for a lifetime license with no hidden fees.
okCommand Line Support - convert emails via command line.
okFast batch conversion - convert multiple emails simultaneously.
okHigh security options - protect your output PDF files (if you choose PDF).
okVarious file naming and date saving options - organize your output files easily.
okVariety of attachment saving options - choose how to save attachments.
okTrust - you can rely on powerful email converters from CoolUtils.
okEasy to use even for beginners - enjoy clear user-friendly interface.

How to convert OST to MBOX?

1

💾 Upload Your File: Go to the site, click on «Upload File,» and select your OST file.

2

✍️ Set Conversion Options: Choose MBOX as the output format and adjust any additional options if needed.

3

Convert and Download: Click 👉«Download Converted File»👈 to get your MBOX file.

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Cloud Based
There is no need to install any software to convert OST to MBOX with CoolUtils. Simply access the internet, and you can easily convert your file online with our service.
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Secure
Our hardware operates in top-tier, highly secure data centers equipped with advanced electronic surveillance and multi-factor access control systems.
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Ease of Use
Converting from OST to MBOX has never been so simple. Our intuitive interface lets you convert files in just a few clicks.
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Privacy
We understand the importance of privacy, which is why we've developed a comprehensive privacy policy detailing how we handle your files and personal information. You can read the policy here.

 

OST File

File extension .OST
Category File
DescriptionOST is an offline data file used by Microsoft Outlook to store emails, calendars, and other information when working in cached mode with an Exchange server.
Associated programsMicrosoft Outlook, SysTools OST Viewer, Kernel OST Viewer
Developed byMicrosoft
MIME typeapplication/vnd.ms-outlook
Useful linksMore detailed information on OST files
Conversion typeOST to MBOX

MBOX File

File extension .MBOX
CategoryDocument File
DescriptionMany of us prefer to store emails in the MBOX format, which organizes all messages into one file. This format is intended for storing email messages on hard drives. In fact, a MBOX file is a long text file, in which messages are organized in a string, each message starting with “From”, which is followed by a space character and the sender’s email address. Initially, MBOX files were used on Unix. These files are supported by Qualcomm Edora, Microsoft Entourage, Thunderbird and Mozilla clients.
Associated programsThunderbird
Developed by
MIME type
Useful linksMore detailed information on MBOX files
5 star2026-02-18
Migrated an old Exchange mailbox to Thunderbird by converting the OST to MBOX. The entire folder structure was preserved and imported without errors.
Lena Bauer
4 star2026-04-02
The conversion handled a 6 GB OST file and produced a valid MBOX that Thunderbird imported cleanly. Took about 20 minutes for the full archive.
Rahul Desai
5 star2026-05-14
Needed to switch from Outlook to a Linux mail client. OST to MBOX worked perfectly and I didn't lose a single email in the migration.
Anna Kowalczyk

Rating OST to MBOX   5 star OST to MBOX   4.7 (211 votes)
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OST to MBOX Converter — Frequently Asked Questions ▼

What is an OST file and why can't I just open it in another mail client?

OST (Offline Storage Table) is Microsoft Outlook's local cache format: a proprietary binary file that stores emails, calendar events, contacts, and tasks for offline use. It is tied to the Exchange/Microsoft 365 account that created it and can only be read by Outlook on the same machine. Other mail clients — Thunderbird, Apple Mail, Postbox, Evolution — do not recognize the format at all, which is why conversion is necessary before migrating.

What email data is preserved when converting OST to MBOX?

The converter preserves message headers (From, To, CC, Date, Subject), the full message body (both plain text and HTML parts), embedded inline images, and file attachments. MBOX clients like Thunderbird will display the converted messages with the original sender, recipient, and timestamp intact. Calendar items and contacts stored in the OST are not email messages and will not appear in the MBOX output.

Which email clients can import the resulting MBOX file?

MBOX is a standard Unix mailbox format supported by Mozilla Thunderbird, Apple Mail, Eudora, The Bat!, Evolution (Linux), and many archiving tools. Thunderbird is the most common import target: use the ImportExportTools NG add-on to import the .mbox file into any local folder. Gmail and Google Workspace also accept MBOX imports via Google Takeout/migration tools.

Will the folder structure from Outlook be preserved in the MBOX output?

MBOX is a single flat file per folder. The converter maps each Outlook folder (Inbox, Sent, custom folders) to a separate MBOX file, preserving the folder hierarchy as individual files. When you import into Thunderbird, each MBOX file becomes a corresponding local folder, so your top-level folder organization is maintained.

What is the maximum OST file size for the online converter?

The online converter accepts OST files up to 50 MB. Personal OST files can range from a few MB to tens of gigabytes depending on how long the account has been in use. Large OST files must be processed with the desktop CoolUtils mail converter, which has no file size limit and can extract specific date ranges or folders rather than the entire archive.

Can I batch convert multiple OST files or automate the migration?

The online converter handles one file at a time. For large-scale migrations — multiple users, full mailbox archives, or automated IT workflows — use the CoolUtils desktop mail converter on Windows 7–11. It processes multiple OST files from the command line, supports folder filtering by date or name, and runs silently as a scheduled task or within a migration script.
 

 

Convert OST to MBOX Online for Free

OST (Offline Storage Table) files are used by Microsoft Outlook to store emails, calendars, contacts, and tasks for offline access. However, these files are only compatible with Outlook, making it difficult to migrate emails to other platforms. If you are switching to an alternative email client like Thunderbird, Apple Mail, or Postbox, converting OST files to MBOX is essential for seamless migration.

MBOX (Mailbox) is a popular file format supported by many email clients, including Mozilla Thunderbird, Apple Mail, and Eudora. By converting OST to MBOX, you can transfer your email messages easily while preserving important details such as attachments, sender and recipient information, timestamps, and message bodies. This makes MBOX a versatile and reliable format for managing your email archives.

Our online OST to MBOX converter is designed to simplify the email migration process. You don't need to download or install any additional software. Everything happens securely in your web browser, ensuring a hassle-free experience. Whether you are archiving old emails, switching platforms, or creating a backup, our tool ensures a fast and accurate conversion process.

To convert OST to MBOX, follow these simple steps:

Our tool ensures that all essential email data is preserved during the conversion, including subject lines, sender and recipient details, timestamps, message content, and even embedded images and attachments. This guarantees that no critical information is lost during the migration process.

Why choose our OST to MBOX converter? Here are some of the key advantages:

Whether you need to migrate emails, create backups, or ensure long-term compatibility with different email clients, our OST to MBOX converter is the perfect solution. It streamlines the email migration process, saving you time and effort while ensuring professional-grade results.

Start your conversion today by visiting our OST to MBOX converter page. Upload your file, let our tool handle the processing, and download a fully compatible MBOX file in seconds. Enjoy the convenience, speed, and security of our free online service from CoolUtils.

Convert from OST

Using CoolUtils, it is possible to convert OST files to a variety of other formats:

Convert to MBOX

Using CoolUtils, it is possible to convert a variety of other formats to MBOX files:

                                                                                                 

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