How to open EML, MSG, and winmail.dat files
Updated July 22, 2026
Someone sent you a file instead of an email, and double-clicking it did nothing useful. Chances are it's one of three things: a .eml, a .msg, or a file literally named winmail.dat sitting where an attachment should be. Each one fails for a different reason, and each one has a different, usually free, fix.
None of these are broken files. A .eml is just a saved copy of an email in a format almost every mail system agrees on. A .msg is Outlook's own private format for the same idea. Winmail.dat is what happens when Outlook tries to send you formatting that your mail app can't read, so it stuffs everything into one wrapper instead. The problem isn't the file. It's what's installed on the machine you're opening it on.
What you really received
A .eml file is plain text under the hood. Open one in a text editor and you'll see readable header lines (From, To, Subject, Date) followed by the message body, all defined by an old, stable internet standard that Gmail, Apple Mail, Thunderbird, and basically everything else can read. This is why .eml is the closest thing email has to a universal export format.
A .msg file is not plain text. It's a compound binary container, the same general file family Windows once used for old .doc and .xls files, with the message properties, body, and attachments packed into internal binary streams. Only software that specifically understands that container format can unpack it, and for a long time that meant Outlook and little else.
Winmail.dat is different again. It's TNEF, short for Transport Neutral Encapsulation Format, a packaging scheme Outlook uses when it sends a message in Outlook Rich Text with formatting or features a plain email can't carry. If the recipient's mail server or app understands TNEF, it unpacks invisibly and you just see a normal email. If it doesn't, everything, message body and every attachment, gets flattened into one attached file named winmail.dat, and the actual email you receive looks empty.
Why double-clicking does nothing
On Windows, a .eml file sometimes opens in Outlook if it's installed, sometimes in whatever mail app came set as default, and sometimes triggers a "how do you want to open this file" prompt with no good answer, because plenty of Windows installs have no default handler registered for it at all. A .msg file is worse: outside of Outlook, Windows usually has nothing registered for it whatsoever.
On a Mac, Apple's Mail app owns the .eml extension and opens it directly, so that one usually just works. But .msg has no native owner on macOS. Double-clicking typically gets you a dialog saying there's no application set to open the file, because Apple never built one in, and Outlook for Mac's handling of .msg attachments has never quite matched Outlook for Windows.
Winmail.dat is the one platform-neutral failure: neither Windows nor macOS has ever shipped a built-in way to open it, because from the operating system's point of view it's just an opaque attachment with no registered file type at all.
Your realistic options, format by format
For .eml, you have the most choices of the three. Any desktop mail client (Mail on a Mac, Outlook, Thunderbird) can open one, though most want you to drag it into an existing mail folder or profile rather than just double-clicking, which is overkill for reading a single message. A browser-based viewer skips that step entirely: drop the file in and read it, no mail profile involved. The Email File Viewer handles exactly this, and it works the same for the other two formats below, so you don't have to figure out which one you're holding before you can read it.
For .msg, your options narrow fast. Outlook for Windows opens it natively, obviously. If your workplace mail runs through a browser version of Outlook, that can sometimes open a .msg attachment too, though getting the file uploaded into a message there just to read it is its own detour. A small number of dedicated desktop viewers exist that read .msg without installing Outlook. Quality varies. A browser-based tool that parses the container directly is usually the least fuss if you just need to read one file someone emailed you.
Winmail.dat is really a formatting mismatch, not a missing viewer problem. If everyone in the conversation uses Outlook connected to the same Exchange or Microsoft 365 organization, TNEF unpacks automatically and nobody ever sees a winmail.dat file. The moment one person is on Gmail, iCloud, or almost anything else, it can surface. A few small free desktop utilities exist specifically to unpack TNEF files if you go looking for one, and a browser-based tool that recognizes the format can unpack the message and attachments the same way, without installing anything.
If this is a single one-off file from someone you'll never hear from again, it's fair to ask whether installing anything is worth it. Reading it once in a browser tab and moving on is a perfectly reasonable choice, and arguably the more sensible one for a lot of people.
Why these files keep showing up
A few situations produce this exact mess over and over. The first is a mailbox export: someone left a job, or IT migrated a mail system, and instead of a live inbox you get handed a folder of individual .eml or .msg files, one per message, generated by an export or migration tool rather than typed by a person.
The second is legal or HR discovery work. When a law firm or investigator collects email evidence, the production is often delivered as a folder of individual message files rather than access to a live mailbox, specifically so each message stands alone with its headers intact. If you've been handed a USB drive or a shared folder full of .msg or .eml files as part of a dispute, this is almost certainly why.
The third is simply that you emailed with, or were forwarded mail from, an Outlook shop. Plenty of companies still run Exchange or Microsoft 365 with older Rich Text defaults on some accounts, and any message from one of those to your Gmail or personal address can turn into a winmail.dat without anyone doing anything wrong.
Turning any of them into a paper record
Once you can actually read the message, whether that's in Outlook, Mail, or a browser tool, saving it as a PDF for a permanent record is the same idea everywhere: use your browser's or app's print function and choose Save as PDF (Windows calls this printer option "Microsoft Print to PDF"; a Mac's print dialog has a PDF button built in). That gets you a page that looks like the email looked.
Where it gets trickier is the .msg and winmail.dat cases, since you can't get to a print dialog until something has actually opened and rendered the message first. That's the practical reason a lot of people end up reaching for a browser-based viewer rather than a standalone TNEF or .msg decoder: you want to go straight from a file you can't open to a PDF you can file away, without an extra conversion step in between.
If the email you're archiving contains account numbers, medical details, or anything else you'd rather not have sitting in a plain PDF, it's worth blacking those out with PDF Toolkit's redact tool once you've saved the PDF, before you file it or hand it off, especially if the reason you're saving it in the first place is a legal or HR matter where that copy might get shared further than you expect.