You open an email expecting a contract, a spreadsheet or some photos, and instead there's one attachment called winmail.dat. Double-clicking it does nothing useful, and your phone says the file type isn't supported. This guide explains what the file is, how to get your attachments out on any device, and the exact settings that stop it happening, which you can forward to the sender.
What winmail.dat is
winmail.dat is a file in TNEF format: Transport Neutral Encapsulation Format, Microsoft's way of sending Outlook-specific information across email. When Outlook sends a message in Rich Text Format (RTF), it packs the formatted text, the attachments and some Outlook data (such as voting buttons or meeting details) into this single attachment. The email's MIME type is application/ms-tnef.
Outlook recipients never notice, because Outlook unpacks it automatically. Everyone else, including Apple Mail, Gmail, Thunderbird, iPhone Mail and most Android apps, sees the container. Usually the real attachments are inside it. Sometimes it holds only the formatted message text.
You may also see it named ATT00001.dat or winmail.dat with an unusual capitalisation. Some servers rename the attachment.
How to open winmail.dat on any device
The simplest way is the winmail.dat opener. It runs in the browser on Windows, Mac, Linux, Chromebook, iPhone and Android, and unpacks the file on your device without uploading it.
InboxVue extracts each attachment with its original name, decompresses the rich-text message body when it's included, and opens embedded emails and meeting requests. If you have the whole email saved as an .eml file, drop that instead and the viewer unpacks the winmail.dat automatically. Our guide on how to open an EML file shows how to save one from Gmail or Outlook on the web.
On iPhone and iPad
- In Mail, touch and hold the
winmail.datattachment and choose Save to Files. - Open Safari and go to
inboxvue.com/winmail-dat-opener. - Tap Choose files, then Browse, and select the saved file.
- Tap an extracted attachment to preview it, or save it back to Files.
On Android
- Download the attachment from the Gmail app or your mail app.
- Open Chrome and go to
inboxvue.com/winmail-dat-opener. - Tap Choose files and select the file from Downloads.
On a Mac or PC
Save the attachment, drag it onto the opener in your browser, and download the files. On Windows, if you have classic Outlook, the original email will usually display correctly there, since Outlook understands TNEF.
How to stop winmail.dat (send these steps to the sender)
The fix is always on the sending side. Forward these instructions to the person or IT team that sends you winmail.dat files.
Classic Outlook for Windows: change the default format
- Choose File → Options → Mail.
- Under Compose messages, set Compose messages in this format to HTML.
- Under Message format, set When sending messages in Rich Text format to Internet recipients to Convert to HTML format (or Convert to Plain Text format).
- Click OK.
Classic Outlook: fix a specific contact
Outlook can remember a "send as Rich Text" preference for an individual address. To clear it, open a new message, type the address, right-click it, choose Open Outlook Properties, and set Internet format to Let Outlook decide the best sending format or Send Plain Text only. Removing the address from Outlook's suggested-addresses list (the arrow or × next to it as you type) clears old cached settings too.
Exchange Online or Exchange Server: disable TNEF for external mail
An administrator can stop TNEF for all external recipients with Exchange PowerShell:
Set-RemoteDomain -Identity Default -TNEFEnabled $false
This makes Exchange convert outgoing messages to standard MIME for every remote domain. To target one domain only, create a remote domain for it with New-RemoteDomain and set -TNEFEnabled $false on that one. Leaving the value unset ($null) lets Outlook's own settings decide.
New Outlook and Outlook on the web
These compose in HTML and seldom produce winmail.dat. If the problem continues after a sender switches to new Outlook, the cause is usually the Exchange setting above.
Why winmail.dat still happens in 2026
Rich Text Format is a legacy Outlook option, but many organisations still have it set as a default, especially on long-standing classic Outlook installations and in templates. Mail passing through some Exchange servers can also be converted to TNEF. It persists because the sender sees nothing wrong: their own Outlook displays the message perfectly.
Troubleshooting
The opener finds no attachments. Some winmail.dat files only carry the formatted text of the message. That's shown in the reading pane. Ask the sender to resend if you expected a file.
The file isn't recognised as TNEF. A real TNEF file starts with a fixed signature. Files named ATT00001.dat that aren't TNEF are usually text that a server split off. The viewer shows their content.
The extracted file won't open. The container doesn't change the files inside it. You'll need an app for the file type, for example a word processor for .docx.
The winmail.dat contains an email. Open it in the viewer; embedded messages are shown as their own emails. If you've been sent an Outlook .msg file instead, use the MSG viewer. For standard email files, use the EML viewer.
Summary
winmail.dat is Outlook's TNEF container, created when a message is sent in Rich Text Format. Your attachments are almost always inside, and a browser-based opener gets them out on any device. To stop it, the sender should switch Outlook to HTML and convert RTF for internet recipients, or their admin should disable TNEF for external domains in Exchange.