A client sends me emails with a file called winmail.dat and I can not open it. There is nothing readable inside. What is going on?
I received a winmail.dat attachment. What is it and how can I open it?
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FQ
Fiona Quinn November 2, 2026
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That file is created by Microsoft Outlook when it sends a message in Rich Text Format (RTF). Outlook packs the formatting and the real attachments into winmail.dat (TNEF format). Non-Microsoft mail programs can not unpack it, so you see only that file.
For you: ask the sender to switch the format. In Outlook they can right click the contact in the To field, choose Outlook Properties, and set Send using to Plain Text or HTML. Or in File > Options > Mail > Compose messages in set HTML, and under Advanced check Rich Text options (Internet format: Convert to HTML format). Then send the attachments again.
If you can not wait, a free TNEF viewer or a program like Thunderbird with a TNEF add-on opens the file and shows the real attachments.
Told the client to set HTML format. Attachments arrive normally now. Thanks.