📖 What is an OLM File?
An OLM file is the data file that Microsoft Outlook for Mac uses to store a mailbox. The letters come from Outlook for Mac, and the file carries the .olm extension. A single OLM file can hold a whole account, its emails, contacts, calendars, tasks and notes, bundled together as an archive.
The format belongs to the Mac version of Outlook alone. It is the Mac counterpart to the PST file that Outlook on Windows uses, and the two are not interchangeable. An OLM file will not open in Windows Outlook, which is the single biggest thing people run into with it.
You usually meet an OLM file when someone exports their Mac Outlook mailbox as a backup, or when they move to a Windows computer and need to carry their old mail across. Because it is a proprietary archive, it is meant to be handled by Outlook or by a tool made for it, not read in a text editor.
Key Characteristics
- An email archive, a whole mailbox
- Outlook for Mac, its native format
- macOS only, not for Windows
- Proprietary, made by Microsoft
Good to Know
- Like a Mac PST, the same idea
- Holds more than mail, contacts too
- Made by exporting, from Mac Outlook
- Needs converting, to reach Windows
⚡ Quick Facts
| File Extension | .olm |
| Full Meaning | Outlook for Mac data file |
| Category | Email archive |
| Created By | Microsoft Outlook for Mac |
| Platform | macOS only |
| Stores | Mail, contacts, calendars, tasks, notes |
| Windows Equivalent | PST |
| Opens In | Outlook for Mac |
| For Windows | Convert to PST first |
| Related Extensions | .pst, .ost, .mbox, .eml |
📦 What an OLM File Holds
An OLM file is not just a folder of messages. It packs an entire Outlook account into one archive, keeping the pieces that make up a mailbox together.
Emails
All the messages in the account, with their folders and any attachments kept in place.
Contacts
The address book, so names and details travel with the mail rather than being left behind.
Calendars
Appointments and events, keeping a record of the schedule stored in the account.
Tasks and Notes
To-do items and notes, rounding out everything the mailbox was keeping track of.
🛠️ How OLM Files Are Made
An OLM file does not appear on its own. It is created when you export a mailbox from Outlook for Mac, usually as a backup or to move the data elsewhere.
Open the Tools menu
In Outlook for Mac, go to the Tools menu, where the export option lives, and choose Export.
Pick what to include
Choose which items to save, such as mail, contacts and calendars, into the archive file.
Save the .olm
Name the file and choose a location, and Outlook writes your selection out as an OLM archive.
📂 How to Open an OLM File
How you open an OLM file depends entirely on which computer you are on. This is where the format catches people out.
| On a Mac | Outlook for Mac opens an OLM file through its Import option, restoring the mailbox back into the program. |
| On Windows | Windows Outlook cannot open an OLM file at all. You first convert it to PST, which Windows Outlook does read. |
| Without Outlook | A dedicated OLM tool can read the archive and show its folders, or export the contents to a more common format. |
👁️ Viewing OLM Without Outlook
You do not always have Outlook for Mac to hand, and you may be on a Windows PC. A dedicated OLM viewer reads the archive on its own and shows what is inside.
Load the archive
An OLM viewer or converter opens the file straight from disk, with no mail program installed behind it.
Browse the folders
The tool lays out the mailbox in its original folder tree, so you can read messages, contacts and the rest.
Export what you need
From there you can save items out to a format your other software reads, or just read them in place.
🔄 Converting OLM to PST
Turning an OLM file into a PST is the usual goal, because PST is the format Windows Outlook understands. Once converted, the mailbox imports into Outlook on a PC.
Load the OLM
Open a converter made for OLM files and add the archive you want to move to Windows.
Choose PST output
Pick PST as the target format, which keeps the folder structure of the original mailbox.
Import into Outlook
Bring the finished PST into Windows Outlook with its Import and Export wizard, and the mail appears.
⚔️ OLM vs PST
OLM and PST do the same job on different sides of the fence. Seeing them side by side explains why a conversion is needed at all.
| Point | OLM | PST |
|---|---|---|
| Belongs to | Outlook for Mac | Outlook for Windows |
| Platform | macOS | Windows |
| Stores a mailbox | ✓ yes | ✓ yes |
| Opens on the other | ✗ no | ✗ no |
In short, they are twins built for opposite platforms. Both store a full mailbox of mail, contacts and calendars, but each opens only in its own version of Outlook, which is exactly why moving from Mac to Windows calls for an OLM to PST conversion.
🌉 The Manual Bridge Method
There is a free, if fiddly, way to move mail from Mac Outlook to Windows Outlook without a converter. It uses an online email account as a bridge between the two.
Add an IMAP account
In Mac Outlook, add an online account such as a Gmail or Outlook.com address set up over IMAP.
Drag your mail across
Move the messages and folders you want into that online account, letting them sync up to the server.
Connect it on Windows
Add the same online account in Windows Outlook, and the mail syncs down, ready to save as PST.
🎯 Why People Need It
Nearly every OLM question comes down to one of a few real situations.
| Switching to Windows | Someone moves from a Mac to a PC and needs their old Outlook mail to come with them. |
| Backing up a mailbox | Exporting to an OLM file keeps a safe copy of an account outside the mail program. |
| Sharing an archive | Passing a mailbox to a colleague or to support, who may then need it in another format. |
| Freeing up space | Archiving old mail to an OLM file and removing it from the live account to lighten it. |
🔀 Related Formats
OLM sits among the mailbox and email formats you meet alongside it.
.pst | The Windows Outlook mailbox file, the format an OLM is usually converted into. |
.ost | An offline cache of a Windows Outlook account, related to PST but synced with a server. |
.mbox | A widely supported mailbox format used by Thunderbird, Apple Mail and many others. |
.eml | A single email saved as its own file, readable by most mail programs. |
.msg | A single Outlook message saved to a file, common on Windows. |
❓ Frequently Asked Questions
It depends on your computer. On a Mac, Outlook for Mac opens an OLM file through its Import option, which restores the mailbox. On Windows, Outlook cannot open an OLM file at all, so you first convert it to PST, the format Windows Outlook reads. A dedicated OLM tool can also read the archive and show its folders without Outlook.
An OLM file is the data file used by Microsoft Outlook for Mac to store a mailbox. It holds emails, contacts, calendars, tasks and notes for an account, bundled as a single archive. It is the Mac counterpart to the PST file on Windows Outlook. The two are not interchangeable, so an OLM file will not open in Outlook on a Windows PC.
You use a converter made for OLM files. Load the OLM into the tool, choose PST as the output format, which keeps the folder structure, and let it convert. You then bring the resulting PST into Windows Outlook using its Import and Export wizard, and the mailbox appears. A free but slower alternative is to sync the mail through an online account.
No, not directly. Windows Outlook uses the PST format and cannot read an OLM file, which belongs to Outlook for Mac. To get the mail into Windows Outlook, you first convert the OLM file to PST, then import that PST. This is the single most common issue people hit when moving Outlook data from a Mac to a Windows computer.
They are counterparts for opposite platforms. OLM is the mailbox file for Outlook on Mac, while PST is the mailbox file for Outlook on Windows. Both store a full mailbox of mail, contacts and calendars, but each opens only in its own version of Outlook. That split is exactly why moving from Mac to Windows needs an OLM to PST conversion.
You export one from Outlook for Mac. Open the Tools menu, choose Export, then select which items to include, such as mail, contacts and calendars. Name the file and pick a location, and Outlook saves your selection as an OLM archive. People usually do this to back up a mailbox or to prepare their data before moving to another computer.
Yes, with a tool built for the format. A dedicated OLM reader or converter opens the archive, shows its folders, and can export the contents to formats like PST, MBOX, EML or PDF. This is useful when you do not have Outlook for Mac available but still need to read the mail or move it into another program.
An OLM file is not a live database sitting in a fixed system folder. It is created only when you export from Outlook for Mac, and it is saved wherever you choose during that export, commonly the Documents folder with a name like Outlook for Mac Archive.olm. Outlook for Mac keeps its working mail in its own internal profile, and the OLM is a separate archive you make on purpose.
Yes. Many OLM converters can write the mailbox out to MBOX, which is a widely supported format used by Thunderbird, Apple Mail and others. This is a good choice when you want to move Mac Outlook mail into one of those programs rather than into Windows Outlook. The converter keeps the messages and folders intact during the change.
It is, as long as you keep the original. A good converter preserves the folder structure and the contents without altering the source OLM file, so your data stays intact. As a sensible precaution, keep a backup of the OLM before converting, and check the resulting PST in Outlook to confirm everything came across before you rely on it.
An OLM file grows with the mailbox it holds, so a long history of mail with attachments can make it quite large, sometimes several gigabytes. There is no small fixed limit built into the format. Very large OLM files simply take longer to export, convert or import, which is worth keeping in mind when moving a big account between computers.
Yes. An OLM file is more than an email archive. Along with the messages and their folders, it stores the account's contacts, calendars, tasks and notes. That is what makes it a complete mailbox backup, since exporting to OLM captures the whole account rather than just the mail, and a good conversion carries all of those items across.
Use a dedicated OLM viewer or converter. These tools read the archive straight from disk, with no mail program installed, and lay out the folders so you can browse the messages, contacts and calendars. This works on Windows as well as Mac, which is the usual way people read Mac Outlook mail on a PC. From the tool you can also export the contents to another format.
Yes. Alongside PST and MBOX, many OLM converters can save the mail as PDF, which turns each message into a fixed document you can read, print or file without any mail program. It is a good choice when you want a permanent, shareable copy of certain emails rather than a working mailbox. The converter keeps the message details and attachments in the output.
Not directly, but through a conversion. Both Apple Mail and Thunderbird read the MBOX format, so converting an OLM file to MBOX lets you import the mail into either program. A converter made for OLM handles this, keeping the folders and messages intact, which makes MBOX the natural bridge when you want Mac Outlook mail in a different mail client rather than in Outlook.
📝 Summary
An OLM file is the data file of Microsoft Outlook for Mac, using the .olm extension, that stores a whole mailbox of emails, contacts, calendars, tasks and notes as a single archive. It is the Mac counterpart to the PST file on Windows Outlook, and the two do not open on each other's platform. You create an OLM by exporting from Outlook for Mac, and you open one by importing it back into Mac Outlook. To use the mail on Windows, you convert the OLM to PST with a converter, then import the PST into Windows Outlook, or use a free IMAP account as a bridge. Converters can also write an OLM to MBOX, EML or PDF for other programs. The whole point of understanding the format is saving Outlook data when moving between a Mac and a Windows PC.