Winmail.dat Opener

Why I built this

Every few months somebody forwards me an email with a file called winmail.dat stuck to it and the same question: where's the PDF? The sender swears they attached a contract or a set of photos. On the receiving end there's just one odd file that nothing wants to open. Double-click it on Windows and you get asked which app to use. On a Mac it does nothing useful. On a phone it often won't even download.

The attachments are almost always in there. They're just wrapped in a format only Outlook understands. I wanted a way to unwrap it that didn't mean installing an old Windows utility or uploading someone's private email to a stranger's server, so I wrote one that runs entirely in the browser.

What winmail.dat actually is

Outlook and Exchange have their own way of packing a message, called TNEF (Transport Neutral Encapsulation Format). It holds the rich text formatting, meeting details and any attachments in one binary container. When the sender's Outlook decides the other side can handle rich text, it sends that container along as winmail.dat. Outlook on the receiving end unpacks it and you never notice. Gmail, Apple Mail, Thunderbird and most phone apps don't, so you get the raw container instead.

Sometimes the file arrives as ATT00001.dat, ATT00002.dat and so on. Same thing, different name, usually because a mail server renamed it on the way. This tool checks the first four bytes of the file (the TNEF signature) instead of trusting the name, so you can drop it in whatever it's called. Even a file with no extension at all works.

How to use it

If you'd rather keep the whole email as one tidy document, save it from your mail app as an .eml file and run it through the EML to PDF converter.

Save the winmail.dat from your email to your computer or phone. Drop it into the box above, or click the box to pick it, then press Open File. A moment later you'll see the subject and date if Outlook stored them, the message text, and a list of every file that was packed inside. Each one has its own Download button. If there's more than one, Download all as ZIP grabs everything in one go.

You can select several .dat files at once. That helps when a whole thread came through broken and every reply carries its own winmail.dat. Each file gets its own card, and the ZIP puts each one in a separate folder so two files called image001.png don't overwrite each other.

Images get a small thumbnail in the list, so you can tell which one is the signature logo and which is the photo you actually wanted. Pictures that were placed inside the message itself, like logos or pasted screenshots, show up in the message the way the sender saw them.

A few real situations

Got a whole Google Takeout mailbox full of old emails like these? The MBOX viewer opens the .mbox file, and any winmail.dat you save from it can come straight back here.

A supplier sends an order confirmation from Outlook and your Gmail shows a 40 KB winmail.dat instead of the PDF. Drop it in and the PDF comes out exactly as they attached it, byte for byte. You can open it, print it, forward it.

A colleague invites you to a meeting and all your phone shows is winmail.dat. Meeting invites get sent this way a lot. The tool shows it as a meeting request, with start time, end time and location when they're in the file, so at least you know when to turn up.

An accountant forwards an old email that had another email attached to it. Outlook packs the inner message inside the outer winmail.dat. Those nested messages appear as Attached email, with their own text and files underneath, and they get their own folder in the ZIP.

Someone on a Mac gets a steady stream of these from one client. You could ask the client to change their Outlook settings (they probably won't). Or you can drop each batch here when it lands, which takes about ten seconds.

Why it runs in your browser

The older winmail.dat sites work by uploading your file to their server, unpacking it there and sending the attachments back. That's fine for a holiday photo. It's not great for a signed contract, a payslip or test results, which is exactly the sort of thing people email from work Outlook accounts. Here the file is read by JavaScript on your own device. Nothing is sent anywhere and there's no account. The only limit is 200 MB per file, and no mail server lets an attachment get that big anyway.

The message body is handled the same careful way. HTML emails are shown in a locked frame with scripts switched off, and images that would load from the internet are blocked. Those remote images are often tracking pixels, and loading them would tell the sender exactly when you opened the message.

What it won't do

Downloaded the .rtf and need a PDF you can file or send on? The RTF to PDF converter handles that.

It can't repair a file that was cut short. If a mail server truncated the email, the attachments near the end may be missing or damaged, and no tool can bring those bytes back. Ask the sender to send it again.

It doesn't open encrypted or digitally signed Outlook messages. Those arrive as smime.p7m, not winmail.dat, and need the recipient's private key.

And sometimes there's honestly nothing to extract. Plenty of winmail.dat files only carry the formatted copy of the message text. The tool says so plainly when a file has no attachments, so you're not left wondering whether you missed something.

The message text is rebuilt from what Outlook stored. Plain text and HTML emails come out the same as the original. Messages that only exist as rich text lose some styling, like fonts and colors, but keep the words and line breaks. If you need the exact layout, download the original .rtf and open it in Word or LibreOffice.

Stopping winmail.dat at the source

If one person keeps sending you these, the real fix is on their side. In Outlook for Windows they can go to File, Options, Mail and set Compose messages in this format to HTML. For one contact only, they can open that contact's email address properties and pick Send using Internet format. Exchange and Microsoft 365 admins can switch TNEF off for outside domains with the Set-RemoteDomain command. Any of those stops winmail.dat for good. Until somebody gets around to it, this page saves a lot of back and forth.