Free online OST to MBOX converter

What the OST format is

OST is Outlook's offline storage table: a local cache of messages, folders, calendar items, contacts, and other Exchange, Microsoft 365, or IMAP account data. Microsoft Outlook creates an .ost file when an account is configured for cached or offline use, normally under the user's profile directory.

An OST is tied to its Outlook profile and account data. It is not a standard interchange format, and copying an OST to another computer does not normally make its contents readable there. A disconnected or orphaned OST may also lack current server data.

What the MBOX format is

MBOX stores multiple email messages in one plain mailbox file, with each message separated by a From_ delimiter line. Thunderbird, Apple Mail, Mozilla-based applications, and many Unix mail tools support MBOX, although they may use different filenames, folder structures, and MBOX variants.

MBOX is useful for transferring mail between supported applications, keeping an application-independent message archive, and processing messages with mail utilities. It generally preserves email headers, body text, and attachments as MIME data without requiring an Outlook profile.

How to actually convert OST to MBOX

The practical method is an OST-aware converter; Outlook itself does not offer a direct File → Export command to MBOX. You can use the built-in converter on 101convert.com without installing software.

1. Make a copy of the original .ost file and work from the copy. Do not modify or delete the source while conversion is running.
2. Open the OST-to-MBOX converter on 101convert.com and select the copied OST file.
3. Choose MBOX as the output format. If the service offers folder selection, select the required mail folders instead of the entire cache.
4. Start conversion and wait for processing to finish. Large OST files can take longer and may produce one MBOX file per mailbox or folder, depending on the converter's output design.
5. Download the resulting MBOX file or files, then import them through the destination mail program's import function. In Thunderbird, an import extension such as ImportExportTools NG is commonly required; Apple Mail can import through File → Import Mailboxes.

For sensitive business mail, use a local desktop OST conversion program instead of uploading the file. A local converter should support the specific OST version, encrypted or damaged files, folder selection, and export to separate MBOX mailboxes.

Quality, compatibility, and limitations

Conversion can preserve messages and MIME attachments but may not reproduce Outlook-specific features such as voting buttons, follow-up flags, Quick Steps, categories, rules, cached autocomplete data, or some calendar and contact records. Verify representative messages, attachments, dates, sender fields, and nested folders after import.

If the OST belongs to an active Exchange or Microsoft 365 account, synchronize Outlook first so the cache contains the newest server data. An orphaned OST cannot always be decoded; encrypted, corrupted, password-protected, or partially synchronized files may require Outlook profile access or specialized recovery software.

Do not rename .ost to .mbox: the internal formats are unrelated. MBOX compatibility also varies by application, so keep the original OST and the exported files until the destination mailbox has been checked and backed up.