Convert an .olm archive to MBOX
MBOX is the format Apple Mail, Thunderbird, Postbox and most Unix mail tools read, and it is the sane destination for an Outlook for Mac archive you want to keep using. Drop the .olm here and you get one .mbox per Outlook folder, with the folder tree intact. The file is read in this browser tab, so a 12 GB archive is as private as one you never opened.
The file you get
A .zip holding one .mbox file per Outlook folder, Inbox.mbox, Sent Items.mbox, Archive.mbox and any subfolders, in the same tree the archive had. Each message inside is a standard RFC 5322 message with its headers, HTML and plain-text bodies and its attachments as MIME parts, so nothing is left behind in a side folder. There is also a single-file option, if the app you are importing into wants one mailbox rather than a tree.
Steps
- 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.
- Drop the
.olmon this page. It is read by JavaScript in your browser tab; the file never leaves your machine. - Untick any folders you do not need. Everything is selected by default; a big archive converts faster if you take the Inbox and Sent Items first.
- Download the zip and unzip it somewhere you can find, for example
~/Documents/olm-mbox/. - Import the folder into your mail app: in Apple Mail, File → Import Mailboxes → Files in mbox format. In Thunderbird, use the ImportExportTools NG add-on into Local Folders. Both routes are written out step by step on the Apple Mail and Thunderbird pages.
What carries over
- Every message with its subject, date, From, To and Cc
- The HTML body as Outlook stored it, plus a plain-text alternative for apps that prefer one
- Attachments inside the message, as MIME parts, not as loose files
- The folder tree, one
.mboxper folder, names unchanged - Message-ID and reply headers where the archive recorded them, so threading survives in apps that thread
Good to know
- 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
.olmformat. Mail, attachments, contacts and calendar entries are read from the XML Outlook writes; unusual items (notes, tasks, journal entries) are skipped in this version. - Apple Mail drops everything into a mailbox called Import in the sidebar. That is normal; drag the folders where you want them afterwards and delete the empty Import mailbox.
- Thunderbird cannot import an mbox into an IMAP account. Import into Local Folders, then drag messages up to the account if you want them on the server.
Questions
Does the folder structure survive?
Yes. Every Outlook folder becomes its own .mbox inside the same directory tree, so Inbox, Sent Items and any subfolders arrive separately rather than as one heap. In Thunderbird, tick "Maintain Folder Hierarchy" in ImportExportTools NG to keep it.
Is there a size limit?
No. The archive is read one entry at a time rather than loaded whole, and in Chrome or Edge the output is written to disk as it is produced. Multi-gigabyte archives work; they just take a few minutes.
Can I get MBOX straight out of Outlook without an .olm?
People on the Apple discussion forums report that dragging a mail folder from a running Outlook for Mac onto the desktop produces an .mbox. It works only while Outlook is installed, licensed and running, one folder at a time, and Microsoft does not document it. If you already have the .olm, this page is faster and covers every folder at once.
Why is this free when the converters on the rest of the page are $29 to $49?
Those are Windows programs with a company behind them. This is one static web page: the conversion runs on your own computer, so serving it costs us nothing beyond the domain, and the ads on the page cover that.
Will the MBOX import into Gmail?
Not directly; Gmail has no mbox import. Import the mbox into Thunderbird, add your Gmail account to Thunderbird as IMAP, then drag the folders across. Thunderbird uploads them.