Import an .olm archive into Thunderbird

The file you get

A .zip of .mbox files, one per Outlook folder, arranged in the same tree. ImportExportTools NG can take the whole directory in one go with the hierarchy preserved.

Steps

1. Get the .olm out of Outlook for Mac first: Tools tab → Export, tick what you want, Continue, save the file. If Export is greyed out you are in the new Outlook; switch the New Outlook toggle off, top right, and it comes back.
2. Drop the .olm on this page. It is read by JavaScript in your browser tab; the file never leaves your machine.
3. Download the zip and unzip it.
4. In Thunderbird, open the menu (three lines, top right) → Add-ons and Themes, search for ImportExportTools NG, and add it. Restart Thunderbird.
5. In the folder pane, right-click Local Folders (not an IMAP account) → ImportExportTools NG → Import mbox file.
6. Choose Import directly one or more mbox files for single files, or the directory option with Maintain Folder Hierarchy ticked to bring the whole tree in at once. Click OK.
7. The Outlook folders appear under Local Folders with their messages and dates.

What carries over

• Messages with headers, HTML body and plain-text alternative
• Attachments, as attachments
• The Outlook folder tree, when you import the directory with "Maintain Folder Hierarchy"
• Original dates, so sorting by date is still meaningful
• Threading, where the archive recorded the reply headers

Good to know

• Mbox cannot be imported into an IMAP folder. Import into Local Folders first; you can drag messages into an IMAP account afterwards and Thunderbird will upload them.
• ImportExportTools NG is a third-party add-on maintained outside the Thunderbird project. Install it from addons.thunderbird.net rather than from a search result.
• Outlook writes times in UTC but leaves the time zone off the timestamp. Every date here is read as UTC, so nothing shifts by your offset, a common fault in other converters.
• Archives over about 2 GB are written straight to disk in Chrome and Edge. Safari and Firefox assemble the download in memory, so on a very large archive use Chrome, or convert one folder at a time.
• Microsoft has never published the .olm format. Mail, attachments, contacts and calendar entries are read from the XML Outlook writes; unusual items (notes, tasks, journal entries) are skipped in this version.