Someone sent you an email that looks empty, with a single useless attachment called winmail.dat. The real message and the real files are inside it. Drop it above and get them back, without installing anything and without uploading your mail to a stranger's server.
Nothing on your computer is registered to open a winmail.dat file, which is why double-clicking it does nothing useful, or dumps a screen of unreadable characters into a text editor. It isn't corrupted and you haven't done anything wrong. It's a packaging mistake that happened before the message ever reached you, and the contents are still perfectly intact inside the file.
This page unpacks it. The file is read in your browser tab on your own machine, so a confidential contract or an HR message stays exactly where it is. That matters more than it sounds: most "winmail.dat opener online" results ask you to upload the file to their server first, which means handing private mail to an unknown third party just to read your own message.
Outlook can send mail in three formats: Plain Text, HTML, and a third one Microsoft calls Outlook Rich Text. That last one is proprietary. To carry its formatting, along with things like voting buttons and meeting details that only Outlook understands, it wraps the message body and every attachment into a single binary container using a format called TNEF, short for Transport Neutral Encapsulation Format. The container conventionally arrives named winmail.dat.
If the recipient is also on Outlook or Exchange, that container is unpacked silently and nobody ever learns it existed. If the recipient is on Gmail, Apple Mail, Thunderbird, a phone mail app, or almost anything else, the container is not understood, so it gets passed through as a plain attachment. The result on your screen is a message with no visible body and one file you can't open, even though the sender attached a perfectly normal PDF and typed a perfectly normal paragraph.
The name "Transport Neutral" is one of the great ironies in email: the format survives transport perfectly and is understood almost nowhere. Because the sender's Outlook chose it at compose time, no setting on your end can prevent it. What you can do is unpack it, which is what this page is for, and then ask the sender to change one setting so it stops happening.
Download winmail.dat the way you'd save any attachment, to your Downloads folder, or to Files on an iPhone. Don't bother double-clicking it afterwards; there's no app on a normal system that will do anything sensible with it.
Drag it in, or tap to pick it. The file is identified by its actual bytes rather than its name, so it still works if your mail server renamed it to something like att00001.dat along the way.
The subject and body appear, along with every attachment the sender actually sent. If the sender's formatted Rich Text body was packed in there, it's decompressed and shown as readable text rather than markup soup.
Download any recovered attachment byte for byte, or turn the unpacked message into a PDF: Print / Save as PDF for an exact-looking copy, or Download PDF for a records-style report with the headers, timestamps and an attachment list.
Thunderbird has some built-in TNEF handling, but it only kicks in when the message is labelled in a way Thunderbird recognises, so plenty of winmail.dat attachments still land unopened. The usual advice is to install an add-on such as LookOut (fix version) and restart. If you'd rather not add an extension to your mail client, right-click the attachment, choose Save As, and drop the file here instead. Same result, nothing installed, and the file never leaves your computer.
Gmail doesn't decode TNEF at all. It shows you an empty-looking message with a winmail.dat file attached, because it genuinely doesn't know what's inside. Download the attachment from Gmail and drop it on this page. Outlook.com and Yahoo Mail behave much the same way when the sender's Outlook used Rich Text.
macOS ships nothing that reads TNEF, so the file just sits there. Drag it from Mail to the Desktop and drop it on this page. This also covers Outlook for Mac, which, despite the name, doesn't always unpack these either.
Save the attachment to Files on iOS or Downloads on Android, open this page in your phone's browser, and tap the drop zone to pick it. There's no app to install. Once a recovered attachment is open you can hand it to another app through the normal share sheet.
If your mail app told you the attachment is a MAPI 1.0 embedded message and is not supported, the container holds an entire email nested inside it, typically a message that was forwarded as an attachment rather than inline. Your mail app managed to peel back the outer layer, found a whole message where it expected a file, and gave up. Dropping the winmail.dat here opens that nested message in place with a breadcrumb trail, so you can read it and pull out its own attachments. Chains several levels deep are handled the same way.
The file isn't named winmail.dat. Some servers rename it to att00001.dat, ATT00002.dat or similar. That makes no difference here, because the container is recognised from its contents rather than its filename.
It turns out not to be TNEF at all. Plenty of unrelated programs write files ending in .dat, and they have nothing to do with email. If you drop one in, you'll be told plainly that it isn't a TNEF container instead of being shown pages of garbage. Knowing which of the two you have is usually the whole problem.
Nothing on your end can prevent this, because the format is decided by the sender's Outlook when the message is written. The fix has to happen there, and it's a one-time setting. It's worth sending them this page, since most people producing winmail.dat files have no idea they're doing it.
Change it for all future mail (the real fix):
Just for one message: while composing, open the Format Text tab and pick HTML or Plain Text before sending.
Just for one contact: open the person in Contacts, double-click their email address, and set it to send in plain text. Handy when a company's internal Rich Text habit only breaks for outside recipients.
The container itself isn't malicious. It's ordinary Outlook packaging and it almost always shows up by accident. But it carries attachments like any other email, so treat what comes out of it with the same caution you'd apply to any attachment from an unexpected sender.
On this page the unpacked message renders inside a locked-down frame with scripts and same-origin access both switched off, so even a hostile message can't execute anything. Remote images stay blocked until you decide otherwise, which stops a sender's tracking pixel from reporting when and where you opened it. Attachment previews are limited to images and PDFs, verified against the file's real bytes rather than its extension; anything else downloads as a file rather than being displayed. Nothing is written to storage, so closing the tab ends the session.
Have a different kind of email file? The full email file viewer opens these too, in the same private way: