Free online OLM to MBOX converter

Mail stored in Outlook for Mac is often needed in Thunderbird, Apple Mail, or another application that accepts MBOX rather than Outlook’s OLM archive. Converting it also provides a portable mailbox file for migration, local backup, or long-term email access.

What the OLM format is

OLM is Outlook for Mac’s proprietary archive format. Microsoft Outlook for Mac creates OLM files when you use Tools → Export to archive mail, contacts, calendars, tasks, and notes. You typically encounter an OLM file when moving data from Outlook for Mac, restoring an old Mac mail archive, or switching to a different email client.

What the MBOX format is

MBOX is a mailbox format that stores multiple email messages sequentially in one file. Thunderbird, Apple Mail, Eudora, and several Unix-based mail tools can import or use MBOX data. It is useful for moving email between applications because it is widely supported and can be copied, archived, or processed with standard mail utilities.

MBOX stores messages and their MIME attachments, but it does not provide a universal container for Outlook contacts, calendars, tasks, or notes. Those data types require separate export formats.

How to actually convert OLM to MBOX

Using the online converter

1. Open the OLM to MBOX converter on 101convert.com.
2. Select the source file with .olm as its extension.
3. Choose MBOX as the output format and start the conversion.
4. Download the generated MBOX file and import it into the target mail application.

This method does not require Outlook or additional software. Upload the archive only when its contents are permitted to leave the computer; use a local converter for confidential or regulated mail.

Importing the result into Thunderbird

1. Install and open Thunderbird, then create or select the destination mail account or Local Folders area.
2. Install the ImportExportTools NG add-on from Thunderbird’s add-on manager.
3. Right-click the destination folder and choose the add-on’s MBOX import command, then select the downloaded MBOX file.
4. Open several messages and attachments to verify the import before deleting the original OLM archive.

Using Outlook for Mac as an intermediate step

Outlook for Mac does not natively export an OLM archive directly to MBOX. It can export individual messages or selected mail as .eml files by dragging them to Finder; Thunderbird with ImportExportTools NG can import those EML files and export a mailbox as MBOX. This workaround is practical for smaller selections, but it is less suitable for a large archive because folder structure and bulk selection may require manual handling.

Quality, compatibility, and limitations

• The converted mailbox normally retains message headers, body text, MIME attachments, and dates, but application-specific flags, categories, rules, signatures, and folder metadata may not transfer exactly.
• MBOX has several mailbox conventions. Thunderbird may store an imported mailbox as a file without an extension inside its profile, while other applications expect an .mbox file or a particular folder layout.
• Large archives can produce large MBOX files and slower imports. Import folders separately when the converter offers folder selection, and keep a backup of the original OLM.
• Check messages with inline images, non-ASCII names, unusual attachments, and nested folders after conversion; these areas expose compatibility differences between mail clients.