The Problem: Apple Mail Cannot Read OST Files
Apple Mail on macOS has no built-in capability to open or import OST files. The OST format is a proprietary Outlook cache format bound to a specific Exchange or Microsoft 365 profile. Even Microsoft’s own Outlook for Mac cannot open OST files created on Windows. If you are switching from a Windows PC with Outlook to a Mac and want your archived emails in Apple Mail, the OST file on your old machine is essentially locked.
This is a frequent issue for professionals switching platforms, companies migrating their workforce from Windows to Mac, or anyone who has recovered an OST file from an old hard drive and needs to access the emails on a Mac.
Solution: Convert OST to EML, Then Import Into Apple Mail
The cleanest migration path is to convert the OST file to EML format — which Apple Mail opens natively — using MailtoPst as an intermediary.
Step-by-Step Migration
- Locate the OST file on the Windows machine. Default path:
C:\Users\<username>\AppData\Local\Microsoft\Outlook\. - Copy the OST file to a USB drive, network share, or cloud storage so you can access it from your Mac (or upload directly from the Windows machine).
- Go to mailtopst.com and upload the OST file. First convert it to PST (the intermediary step), then convert the PST to EML via PST to EML. Alternatively, you can chain the conversion in one step if the tool supports it.
- Download the EML files — you receive a ZIP archive with one EML file per message, organized in folders.
- Unzip on your Mac and navigate to the folder containing the EML files.
- Open Apple Mail and create the folder structure you want (Inbox, Archive, specific project folders, etc.).
- Select EML files in Finder and drag them into the target Apple Mail folder. Apple Mail imports them immediately, preserving subject lines, dates, attachments, and formatting.
- Verify — open a few messages to confirm that HTML formatting, inline images, and attachments display correctly.
Alternative: Use MBOX for Bulk Import
If you prefer to import everything at once rather than handling individual EML files, convert to MBOX format instead. Apple Mail can import MBOX files via File, then Import Mailboxes, then select “Files in mbox format.” This is faster for large archives but gives less granular control.
Why MailtoPst for OST to Apple Mail Migration
- Two-format flexibility — choose EML for granular per-message control or MBOX for bulk import.
- Complete extraction — emails, attachments, folder hierarchy, read/unread status, and timestamps are all preserved.
- No Windows tools needed — you can upload the OST from any platform. The conversion happens in the cloud.
- Secure processing — files encrypted via TLS, processed on EU servers, auto-deleted after conversion. GDPR compliant.
- Free trial — try free with no credit card required. Test the conversion with 100 MB of free credit.
Upload your file now and move your Outlook emails to Apple Mail.
Tips for a Smooth Mac Migration
- iCloud synchronization — if your Apple Mail is connected to iCloud, imported emails may sync to iCloud and count against your storage quota. Consider importing into “On My Mac” folders to keep them local.
- Large OST files — OST files from long-time Outlook users can be 10 GB or more. Ensure your Mac has enough disk space for both the downloaded EML/MBOX files and the Apple Mail import.
- Contacts and calendar — OST files also contain contacts and calendar events. These are not part of the EML export. Use a vCard or ICS export from Outlook (or the Exchange server, if accessible) to migrate those separately.
- Search indexing — after importing a large number of emails, give Spotlight a few hours to index the new messages before expecting full search results in Apple Mail.
Frequently Asked Questions
Can I open an OST file directly on a Mac?
No. macOS has no native support for OST files, and even Outlook for Mac cannot open Windows OST files. You must convert the OST to EML, MBOX, or PST first.
Will the folder structure from Outlook be preserved?
Yes. When you convert to EML or MBOX with MailtoPst, the folder hierarchy from the OST is maintained. You can recreate the same structure in Apple Mail before importing.
Can I migrate contacts and calendar events too?
The EML/MBOX conversion covers email messages only. For contacts, export them from Outlook as vCard (.vcf) files and import into macOS Contacts. For calendar events, export as ICS files and import into macOS Calendar.
How long does the entire migration take?
The conversion step typically takes a few minutes for files under 2 GB. Importing into Apple Mail is fast for EML files (seconds to a few minutes for thousands of files). MBOX import may take longer for very large archives.
What if the original Exchange account is deactivated?
MailtoPst reads the OST file directly at the binary level, regardless of whether the Exchange account still exists. Orphaned OST files are fully supported.
Upload your file now and enjoy your Outlook emails in Apple Mail.