What is an OLM File And What is Actually Stored in One

An OLM is a ZIP file with a different extension. Inside it every message has been rewritten as an XML document, which is the decision that separates this format from all the others. Formats like EML hand you the message itself. An OLM hands you Outlook for Mac's account of what the message was.

The practical consequences follow from that single difference. It explains why Windows Outlook refuses one, why conversion behaves the way it does and why the file is unexpectedly easy to look inside for a format with a reputation for being closed. If you are not yet sure this is what you are dealing with, the byte sequence PK at the start of the file is the tell. The other mail formats announce themselves quite differently.

An OLM Is a ZIP And You Can Look Inside

Copy the file, rename the copy to end in .zip and unzip it. Nothing about the format resists this, because it is an ordinary archive underneath.

There is no password on an OLM. Outlook for Mac offers no option to set one, which is a rare case of a format being honest about what it protects. A PST does offer a password and it protects nothing either, so the practical position is identical. Anything confidential needs the file or the disk encrypted separately.

Look, but do not edit. Repacking a modified archive is a good way to produce something no importer will touch. The container is ordinary. What is inside it expects to be read by software that knows the layout. No specification for that layout has ever been published.

Every Message Is XML Rather Than a Message

Open one of the files under Accounts and you do not find headers and a body. There is no MIME structure and nothing conforming to RFC 5322. You find an XML document with named elements holding the sender, the subject, the date and the text.

Every kind of Outlook item gets the same treatment with its own family of element names. A contact carries OPFContactCopyDisplayName. A calendar entry carries OPFCalendarEventCopySummary. Tasks, notes and journal entries each have their own set again. So an OLM is an archive of Outlook items rather than a mail archive, which matters when a converter assumes everything inside is an email.

Received: from mail...
From: payroll@example.org
Subject: March statement
Content-Type: text/html

<p>Please find...

<Email>
  <OPFMessageCopySubject>
  <OPFMessageCopySenderAddress>
  <OPFMessageCopyToAddresses>
  <OPFMessageCopySentTime>
</Email>

One thing the XML does get right is character encoding. An XML document declares its own encoding at the top, so accented and non Latin text survives a move between machines without the guesswork that catches out older binary formats.

Conversion out of an OLM is therefore reconstruction rather than translation. A converter reads the XML and builds a fresh message from the pieces it finds. That message is a faithful account of what the export recorded, which is not the same thing as what arrived in the mailbox.

What survives depends on which elements the export wrote. Named fields like subject, sender and time are reliably there. Routing history and authentication results depend on whether the exporter recorded them at all. A rebuilt message cannot carry a signature that was never written down. For everyday reading that costs nothing. Where a message has to be shown to be genuine, source the original from the server rather than from the archive.

Why It Only Exists If Somebody Made One

People go looking for their OLM file and cannot find it. Nothing has gone wrong when that happens.

Outlook for Mac keeps its live data in a different format called OLK, not in an OLM. The archive is written only when a person opens the Tools menu, chooses Export and picks a destination. There is no default folder because there is no automatic export.

That is also why searching for one rarely helps. Finder and Spotlight will find an OLM if somebody saved one. If nobody did they return nothing at all, which looks identical from the desktop.

Mail in On My Computer folders exists nowhere else. An OLM containing them may therefore be the only copy anywhere, which makes the file people cannot find the one they most needed.

What an OLM Can Still Be Imported Into

The format is not a dead end on every client. Legacy Outlook for Mac, meaning the 2011 release that introduced the format and the 2016, 2019 and 2021 versions that followed, reads one back through Tools then Import, choosing the Outlook for Mac archive option. Everything that comes in lands under On My Computer, sorted into Mail, Calendar, People, Tasks or Notes according to what each item is.

That round trip is the job the format was designed for. Export from one Mac. Import on another. The archive behaves exactly as intended.

Exporting takes nothing away. An export writes a copy and leaves the mailbox exactly as it was, which is worth knowing before somebody tidies up afterwards on the assumption that the mail has moved. It also means an OLM can be made from a selection rather than the whole mailbox, so two archives from the same account may hold quite different things.

Why Windows Outlook Refuses One

The ability was never built. What follows is why nobody in either camp is waiting for it.

The two products were built by separate teams on separate foundations, which is why the formats have nothing in common beyond both holding mail. Going the other way is no better, since Outlook for Mac imports PST but the reverse path was never added.

Why the New Outlook for Mac Closed the Door

The Mac side might reasonably be expected to keep the format alive. It has already done the opposite.

Microsoft was asked about this directly and answered plainly. The new client will not import an OLM. It drops On My Computer folders entirely, because it expects every message to live in the cloud. There are no local folders for an imported archive to land in, so the structure inside the file has nowhere to go even if the import existed.

Both doors are shut on current software. The usual framing is that OLM is a Mac format Windows will not read. What is actually true is that the current Mac client will not read one either. The legacy client still does, which is why so many people keep an old install alive purely to open archives. Once that install goes, the only readers left are third party.

What to Do With an OLM You Are Holding

1. Find out whether it holds On My Computer mail. Everything else in the archive probably still exists on a server somewhere. That portion does not, which makes it the part worth protecting first.
2. Unzip a copy to see what is in there. The folder tree under Accounts and Local tells you the shape of the archive in a couple of minutes, without committing to any conversion.
3. Decide what the destination needs. Mail heading for Windows Outlook has to become PST. Anything landing in Thunderbird or Apple Mail needs MBOX. Gmail and Microsoft 365 accept an upload of individual messages. A document review platform wants them one to a file as EML.
4. Expect rebuilt messages rather than original ones. Nothing can restore headers the export never recorded, so treat the archive as a readable copy rather than as the record itself.

If the job is finding out what is actually in the file, our browser based viewers parse it on your own machine with no Mac and no Outlook involved. Where the contents have to end up somewhere usable, the mail converters handle the routes out of this format.