How to Open an Outlook .msg File on a Mac (Without Uploading Private Emails)
If a colleague on Windows drags an email out of classic Microsoft Outlook and sends it to you as an attachment—or saves it into a shared folder—it arrives with a .msg extension. When you double-click that .msg file on a Mac, macOS either says no application is set to open it or launches the wrong program.
You can view the message text and extract attachments from a .msg or .eml file immediately in your browser with the free local .msg & .eml viewer—without uploading your email to a server.
Why Apple Mail can’t open .msg files natively
Email clients on macOS, iOS and Linux standardise on .eml (RFC 5322 / MIME), which stores email headers and multipart attachments as structured text.
Classic Outlook for Windows uses a completely different container when saving individual messages:
- OLE2 Compound File Binary Format (
D0 CF 11 E0 A1 B1 1A E1): A.msgfile is a miniature filesystem inside a single file—the same outer container used by legacy Office.docand.xlsdocuments. - MAPI Property Streams: Inside the OLE2 directory, Outlook stores each email field as a numbered MAPI stream:
__substg1.0_0037001F— Subject line (UTF-16LE)__substg1.0_0C1A001F— Sender display name__substg1.0_0E04001F— Recipient display list__substg1.0_1000001F— Plain-text message body__attach_version1.0_#00000000— Sub-storage for each attached file, holding its filename (__substg1.0_3707) and raw binary bytes (__substg1.0_37010102)
Because Apple Mail only parses .eml and .mbox files, it does not read OLE2 MAPI streams.
.msg vs .eml vs winmail.dat
| File type | Magic bytes (hex) | What it is | Best tool |
|---|---|---|---|
.msg | D0 CF 11 E0 A1 B1 1A E1 | Single message saved from Windows Outlook (OLE2 MAPI container) | Open .msg & .eml viewer or File Sniff for Mac (.msg → .eml) |
.eml | Plain text (From:, Date:, MIME-Version:) | Standard RFC 5322 email message supported by Apple Mail, Thunderbird and Outlook | Apple Mail or Open .msg & .eml viewer |
winmail.dat (.dat) | 78 9F 3E 22 | Transport Neutral Encapsulation Format (TNEF) rich-text attachment wrapper | Open winmail.dat extractor |
If you aren’t sure which one you have, drop the file into the free file type checker to inspect its magic bytes.
Why you shouldn’t upload .msg files to random cloud converters
Forwarded .msg files frequently contain contracts, invoices, legal threads, HR documents or personal identification. Uploading a .msg file to a server-based converter hands the entire email body, headers and every enclosed attachment to a third party.
Instead:
- In any browser: Use the File Sniff
.msg&.emlviewer to read the message and save its attachments locally in memory on your device. - On your Mac: The native File Sniff app for Mac converts
.msgfiles into standard.emlfiles on-device so they open directly in Apple Mail with Quick Look and Spotlight indexing.