OLM to MBOX Converter

MBOX is the plain, open format almost every mail program understands. Thunderbird, Apple Mail, Evolution and many archiving tools import it, and it is the format Google Takeout uses for Gmail exports. Moving out of Outlook for Mac usually means getting from .olm to MBOX, and the common answer is a paid desktop converter that wants your whole mailbox on its servers or on your disk twice. This converter does it in the browser instead, with nothing to install and nothing sent anywhere.

Each folder in the archive becomes one .mbox file named after the folder, such as Inbox.mbox. Every email is rebuilt as a standard message with its original From, To, Cc, Bcc, date, subject and Message-ID, a plain text part, the formatted HTML part, and every attachment encoded inside, so nothing depends on the original archive any more. Lines that start with From are escaped the mboxrd way so importers do not split messages in the wrong place. The ZIP also holds contacts.vcf, calendar.ics with events and tasks, notes.txt and messages.csv, a spreadsheet list of every email.

To get the mail into Thunderbird, install the free ImportExportTools NG add-on and import the MBOX files into a local folder. Apple Mail imports MBOX from File, Import Mailboxes. To reach Gmail or Outlook.com, add that account to Thunderbird and drag the imported folders across. If you only need a few emails, open them and use Download .eml instead, which opens directly in Outlook for Windows and Apple Mail. Before you delete the original archive, open a few of the converted folders in your new mail app and check that the message count, dates and attachments match what you see on this page.

The conversion runs entirely in your browser. The archive is read from your disk in small pieces by a worker in this tab, and the MBOX files are written on your device. No copy of the mailbox, the file name or any email is sent to TaskNinja or anyone else, and nothing is kept after you close the page.

MBOX keeps the email content, attachments and dates, but not Outlook-only details such as category colours, flags or follow-up reminders. Events that repeat are saved with their first date only. Very large archives are best converted one folder at a time, because a single ZIP made in a browser stops at about 4 GB.