--- title: "OLM: The Outlook for Mac Format Almost Nothing Can Open" description: "An OLM file is what Outlook for Mac exports — and Outlook for Windows cannot read it. It is also a ZIP archive full of XML, which is the detail that gets your mail back. Three routes to MBOX, one of them free." url: https://mboxviewerpro.com/blog/olm-the-outlook-mac-format-nobody-can-open/ language: en updated: 2026-10-13 source: Mbox Viewer --- [All posts](https://mboxviewerpro.com/blog/) `formats` `pst` `macos` `archives` # OLM: The Outlook for Mac Format Almost Nothing Can Open An OLM file is what Outlook for Mac exports — and Outlook for Windows cannot read it. It is also a ZIP archive full of XML, which is the detail that gets your mail back. Three routes to MBOX, one of them free. [David Carrero](https://mboxviewerpro.com/author/) · October 13, 2026 An OLM file is the archive Outlook for Mac writes when you use its Export command. It is not a PST with a different extension: different format, different codebase, and Outlook for Windows cannot open it. Neither can Apple Mail, nor Thunderbird, nor any mail reader you are likely to already have. That asymmetry — Microsoft’s Windows client refusing Microsoft’s Mac archive — is what catches people out. The good news is that an OLM is far less opaque than a PST, and you can look inside one right now with nothing installed. ## Outlook for Mac writes OLM, Outlook for Windows writes PST, and neither reads the other [PST](https://mboxviewerpro.com/glossary/pst/) is the Windows side: a proprietary database Outlook uses as its working store. [OLM](https://mboxviewerpro.com/glossary/olm/) is what Outlook for Mac produces when you ask it to export — messages, attachments, contacts and calendar items in one container. Same purpose, and not interchangeable in either direction. Hand a PST to Outlook for Mac and it will import it. Hand an OLM to Outlook for Windows and there is no import path for it at all. In practice the only program that reads an OLM reliably is the one that wrote it. OLM is not even Outlook for Mac’s working format. In the Outlook 2016–2021 and Microsoft 365 generation, live mail sits under `~/Library/Group Containers/UBF8T346G9.Office/Outlook/Outlook 15 Profiles/Main Profile/Data/` as a database plus thousands of small message files. OLM only comes out of the Export command. ## You get handed one on your last day and find out years later The pattern repeats. You are closing an account, handing a laptop back, or moving off the Mac. Outlook offers **Tools → Export…**, the dialog offers exactly one format, and you take it because nothing else is on offer. The `.olm` goes onto a drive labelled “old work mail” and you consider the matter handled. Three years on you need one invoice out of it, and there is no Mac with Outlook any more. The file that was supposed to be your backup needs the exact software you no longer have. ## Rename it to .zip and you can read what is inside This is the part almost nobody mentions: an `.olm` file is a ZIP archive. Work on a copy, rename `archive.olm` to `archive.zip`, and unzip it — Archive Utility on macOS and File Explorer on Windows both open it without complaint. What comes out is a folder tree, one branch per account, mirroring the folder names you had in Outlook. The messages are XML files, several messages to a file, with subject, sender, recipients, dates and body stored as elements. HTML bodies sit in there as escaped text, so a text editor shows tag soup — complete tag soup, nothing missing. Attachments are not encoded inside the XML: they are ordinary files in their own folder, referenced by the messages that carried them. Two useful consequences. If every conversion route fails you can still recover content by hand, and for one missing invoice that is often the fastest answer: unzip, search the tree for the sender’s name, read the XML. And if what you want is the PDF rather than the email around it, the attachments are already sitting there as files. It also explains why OLM converters vary so much in quality. Turning that XML into [MBOX](https://mboxviewerpro.com/glossary/mbox/) is a matter of care, not cryptography: rebuild proper MIME messages, reattach the attachments, keep the structure. ## Route 1: back into Outlook for Mac, out again as EML If you can still reach any Mac running Outlook, this route costs nothing. Import the OLM through **File → Import**, choosing the Outlook for Mac archive option. When the folders appear, select messages and drag them into a folder in Finder: Outlook writes them out as `.eml`, one standard [EML](https://mboxviewerpro.com/glossary/eml/) file per message, headers, body and attachments intact. A folder of EML files is not MBOX, but it is an open format plenty of readers handle, Mbox Viewer among them. The limits are real: you drag in batches, not 40,000 messages at once, and you rebuild the folder structure yourself. ## Route 2: an IMAP account as the bridge to MBOX Better for a whole archive. Import the OLM into Outlook for Mac, add any [IMAP](https://mboxviewerpro.com/glossary/imap/) account — a spare mailbox on your own domain does the job — and drag the imported folders onto it. Outlook uploads everything, hierarchy included. Then point Thunderbird at the same account, let it sync, copy the folders into Local Folders and export them with the ImportExportTools NG add-on. Thunderbird writes MBOX, so what comes out is a plain file any reader opens. [Our guide to converting Outlook archives](https://mboxviewerpro.com/guides/convert-outlook-pst-olm-to-mbox/) covers the Thunderbird half step by step. Give the upload time — servers throttle large batches — and check the account has room first. ## Route 3: paid converters, and what you are actually buying Dedicated OLM-to-MBOX converters exist for Mac and Windows. They are commercial products: expect a one-off licence, and expect the free trial to convert a handful of messages per folder — enough to check the output, not enough to finish the job. What the money buys is the work routes 1 and 2 make you do by hand: folder hierarchy preserved, attachments reattached, no Outlook and no mail account needed. Worth it at two hundred folders, a poor deal at three. ## The lesson is worth more than the file OLM is the cleanest argument there is for archiving in an open format. Nothing about it was malicious. It is the *export* format — the one you are pointed at precisely when you are saving mail for the future — and it fails at the only job an archive has: being readable later, without the vendor. If you are choosing today where a decade of mail will live, that is the whole case in one file. [MBOX, EML, PST and OLM side by side](https://mboxviewerpro.com/blog/email-archive-formats-explained/) compares the options. ## Once you have the MBOX Compare the message count against what Outlook showed you. A conversion that silently drops a folder is the failure worth catching early, and the count catches it. [Opening an MBOX file on a Mac](https://mboxviewerpro.com/guides/how-to-open-mbox-file-on-mac/) covers the reading half; mailboxes up to 1 GB open in Mbox Viewer without buying anything, and the file is only read, never written. Keep the original `.olm` as well. It costs a few gigabytes and it is the only copy that still has the calendar and contacts in it. ## Open your archive with Mbox Viewer Native Mac and Windows app. Streams MBOX and EML files of any size, fully offline. [Mac App Store](https://apps.apple.com/app/mbox-viewer-pro/id6759237715) [Microsoft Store](https://apps.microsoft.com/store/detail/9NW3GVFG7DDB)