Apple Mail cannot open an OST file directly, because OST is a Windows Outlook format. The good news is that Apple Mail imports MBOX natively through File then Import Mailboxes, so the only real work is converting the OST to MBOX first. The fastest route is a converter that turns the OST into Mac-ready MBOX in one step, including orphaned OST files that no longer have a working Outlook profile. The free alternative needs a Windows PC with Outlook to produce the MBOX, then you copy it to the Mac and import it. Either way, the final Apple Mail import is free and built in. Contacts and calendar move separately as vCard and ICS.
Why Opening an OST on a Mac Is Not Direct
An OST file is a Windows Outlook artifact. Outlook creates it as a synced offline copy of an Exchange, Microsoft 365 or IMAP account, bound to the specific Outlook profile that made it. There is no way to open OST on Mac natively: no Mac application reads the format, and Apple Mail is no exception.
Apple Mail stores its mail in MBOX, the same plain text mailbox format used across many email clients. That is the key that unlocks this whole task. Apple Mail can import MBOX directly, with no third-party software, straight from its own File menu. So the problem is never the Apple Mail side. The problem is getting your OST data into MBOX, because nothing on a Mac can read the OST to begin with.
That single fact decides everything below. If you have only the OST file and a Mac, you cannot use the free routes, because they all need a Windows machine with Outlook to read the OST first. A direct converter is what closes that gap.
The Task Has Two Parts
Breaking this into two stages makes it far less confusing than most guides make it sound.
Part 1: OST to MBOX. This is the hard part, and the only part that needs a real tool. The OST has to be read and its mail written out as MBOX. This either needs Windows and Outlook (the free route) or a converter that reads the OST format directly.
Part 2: MBOX into Apple Mail. This is the easy part and it is always free. Apple Mail has a built-in MBOX importer. Once you have an MBOX file on the Mac, this step takes a couple of minutes.
These two parts are the whole job of moving OST to Apple Mail. The methods below differ only in how they handle Part 1. Part 2 is the same for everyone, so it has its own section near the end.
Which Route Fits Your Situation
| Your situation | Best route | Needs a Windows PC? |
|---|---|---|
| You only have the OST file and a Mac | Method 1: Direct converter | No |
| The OST is orphaned, corrupt or encrypted | Method 1: Direct converter | No |
| You still have a Windows PC with Outlook and want a free route | Method 2: Windows route | Yes |
| You already have an MBOX file | Skip to the Apple Mail import | No |
Method 1: Convert OST to MBOX for Apple Mail (Recommended)
This is the route to use if you are working on a Mac, or if the OST is orphaned or inaccessible. To convert OST to Apple Mail on a Mac, a dedicated converter reads the OST format directly and writes MBOX that Apple Mail imports, with no Windows PC and no Outlook involved.
The advantage over the free route is not only speed. Reading the OST once and writing MBOX once keeps the data intact. The free route runs the mail through several conversions (OST to PST, then PST to MBOX), and each hop is a chance to lose folder structure, attachments or non-Latin characters.
The Univik OST Converter opens an OST file on its own and exports it to MBOX ready for Apple Mail, preserving the folder structure, attachments and message details. It reads orphaned and inaccessible OST files that the free route cannot open at all, and it converts in bulk rather than one mailbox at a time. For the full feature set and download, see the OST Converter page.
On a Mac with just an OST file? Convert it straight to Apple Mail-ready MBOX, including orphaned and inaccessible files, with the folder structure and attachments kept intact. No Windows PC, no Outlook.
That is all it takes to import OST to Apple Mail without a Windows machine. Once the converter has produced the MBOX file, jump to the Apple Mail import section. That part is identical no matter how you made the MBOX.
Method 2: The Free Windows Route
If you still have a Windows PC with Outlook and the OST is attached to a working profile, you can produce the MBOX for free. It is multi-step and slower, but it costs nothing. The catch is the requirement: this route cannot run on a Mac alone, because the OST can only be read on Windows.
Stage 1: OST to PST in Outlook
Export the mailbox to PST. In Outlook on Windows, go to File then Open & Export then Import/Export then Export to a file then Outlook Data File (.pst). Pick the folders and finish. The OST data is now in a portable PST.
Stage 2: PST to MBOX
Turn the PST into MBOX. The common free path is to import the PST into Mozilla Thunderbird on the Windows PC, then use the ImportExportTools NG add-on to export the folder as MBOX. This is the fiddly stage, since it means installing Thunderbird and an add-on purely to transcode the file.
Copy the MBOX to your Mac. Move the resulting MBOX file across with a USB drive, AirDrop, a shared folder or cloud storage. Now continue to the Apple Mail import below.
A quicker free shortcut, if you have Outlook for Mac
If the account is set up in the classic (legacy) Outlook for Mac, you can skip Thunderbird. Drag a mail folder from Outlook onto the Finder or Desktop and it saves as a .mbox file, ready for Apple Mail. Two catches here. The redesigned new Outlook for Mac removed this feature, so switch to legacy from the Outlook menu first. And a single drag does not keep nested subfolders, so drag one folder at a time. This route still needs Outlook with the account synced, so it does not help with an orphaned OST.
Why this route fails for many people
It needs a Windows PC, a working Outlook profile and a healthy OST. It cannot touch an orphaned OST (where the original profile is gone), a corrupt OST or an encrypted one. If any of those apply, or you simply do not have a Windows machine, Method 1 is the only route that works.
Importing the MBOX into Apple Mail
This is the easy half, and it is the same for both methods. Apple Mail imports MBOX through its own File menu, with no extra software. The steps below match Apple’s official process on current macOS (Ventura, Sonoma and Sequoia).
Open Mail and start the import. In Apple Mail, choose File then Import Mailboxes from the menu bar.
Choose “Files in mbox format”. In the source list, select Files in mbox format (this is the option for mail coming from Windows or another app) and click Continue.
Select the folder holding your MBOX. Point the importer at the folder that contains the MBOX file, then click Choose and Continue.
Find your mail under “Import”. Apple Mail places the imported messages in a new mailbox called Import in the sidebar. Drag the folders or messages from there into your own mailboxes, then delete the Import mailbox once you are done.
Apple Mail Import Gotchas Most Guides Skip
Apple Mail’s MBOX import is reliable, but it has a few quirks that competing guides rarely mention. Knowing them saves an hour of confusion.
Import fails or comes back empty
Import works
The 2 GB size limit. Apple Mail’s built-in importer struggles with MBOX files larger than about 2 GB, often failing silently or importing only part of the mail. Split a large mailbox into smaller MBOX files, or use a converter that outputs per-folder MBOX files rather than one giant file.
“No mailboxes were found to import.” This error appears even on valid MBOX files, usually because you pointed the importer at the file instead of the folder that contains it. Select the enclosing folder, not the MBOX file itself.
The mail looks missing after import. It is not. Apple Mail puts everything in a mailbox literally named Import, at the bottom of the sidebar. People assume the import failed when the mail is simply sitting in that one place.
Leave enough free disk space. Apple Mail copies the imported data into ~/Library/Mail/ and indexes it. Keep free space of at least the MBOX size, ideally double. Otherwise the import can stall.
Contacts and Calendar
The methods above move mail. Contacts and calendar live in the OST too, but MBOX carries messages only, so they move separately.
Contacts. Export them to vCard, then import the vCard into the Mac Contacts app, which Apple Mail reads from. A converter that exports OST contacts to vCard handles this for an orphaned OST where Outlook is not available.
Calendar. Export to ICS and import that into the Mac Calendar app. As with contacts, a direct converter is the route when Outlook can no longer open the file.
Frequently Asked Questions
Can Apple Mail open an OST file directly?
No. OST is a Windows Outlook format and no Mac application reads it natively, Apple Mail included. You first convert the OST to MBOX, which Apple Mail imports through File then Import Mailboxes. The conversion is the only part that needs a dedicated tool.
How do I open an OST on a Mac without Windows?
Use a converter that reads the OST format directly and outputs MBOX, then import that MBOX into Apple Mail. The free routes will not work on a Mac alone, because they need a Windows PC with Outlook to read the OST first. A direct converter is the only option when you have just the OST file and a Mac.
Does Apple Mail support MBOX import?
Yes. Apple Mail imports MBOX natively. Choose File then Import Mailboxes, select Files in mbox format and point it at the folder holding your MBOX. This is Apple’s official, built-in method and needs no third-party software. The imported mail appears in a mailbox called Import.
Why does Apple Mail say “no mailboxes were found to import”?
Usually because you selected the MBOX file instead of the folder that contains it. Apple Mail’s importer wants the enclosing folder. Put your MBOX file inside a folder, then point the importer at that folder. The error can also appear with corrupt or oversized MBOX files above roughly 2 GB.
Where did my email go after importing into Apple Mail?
It is in a new mailbox called Import at the bottom of the Apple Mail sidebar. The import does not merge mail into your existing mailboxes automatically. Drag the folders and messages from Import into your own mailboxes, then delete the Import mailbox.
Can I move OST contacts and calendar to my Mac too?
Yes, but separately from the mail. Export contacts to vCard for the Mac Contacts app and calendar to ICS for the Mac Calendar app. MBOX only carries messages, so contacts and calendar need their own export. A direct OST converter can produce vCard and ICS for an orphaned file where Outlook is unavailable.
Conclusion
Opening an OST in Apple Mail comes down to one real obstacle: getting the Windows OST into MBOX, which is the only format Apple Mail can read. The Apple Mail side is free and built in, so do not let any guide convince you the import itself is the hard part.
Moving OST to Mac is mostly about that one conversion. If you have a Windows PC with Outlook and a healthy OST, the free route produces the MBOX at the cost of several steps. If you are on a Mac with just the OST file, or the file is orphaned or inaccessible, a direct converter that reads the OST and writes Apple Mail-ready MBOX is the route that actually works. Either way, contacts and calendar move separately as vCard and ICS.