What Is winmail.dat and How to Open It
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If an email arrives with an attachment called winmail.dat (or ATT00001.dat) instead of the files you expected, the files are usually inside it. Open it below to get them out.
Why you got a winmail.dat file
Outlook can wrap a message in a Microsoft-only package called TNEF (Transport Neutral Encapsulation Format). Microsoft's own guide, How message format affects email messages, explains that:
- Messages written in Rich Text format are sent with TNEF. HTML and plain-text messages are not, unless an Outlook-only feature needs it.
- Features that need it include meeting requests, voting buttons, task requests and embedded objects.
- Mail apps that don't understand TNEF (Apple Mail, Gmail and many others) show the package as winmail.dat or a generic name like ATT00008.DAT.
The real attachments, and sometimes the formatted message text, are inside that package. The viewer unpacks it.
How to open it
- Save the winmail.dat attachment from your email app.
- Drop it on the box above.
- Click Download next to each file inside.
If the winmail.dat came inside a saved .eml or .msg file, open that file instead and click Open next to winmail.dat.
How the sender can stop sending winmail.dat
Send these steps to the person whose emails arrive this way. They come from Microsoft's guide:
- For all new emails: in Outlook, File › Options › Mail › Compose messages in this format › choose HTML › OK.
- For one email: on the Format Text tab, choose HTML under Message Format.
Their mail server can also be set to remove TNEF during delivery; that's a job for their IT administrator.
Is winmail.dat dangerous?
The file itself is just a container. What matters is what's inside. Open the attachments only if you were expecting them from that sender. This viewer never runs anything from the file; it only lists and saves the contents.