winmail.dat: what it is and how to open it
An attachment nobody sent on purpose, and the real files are hidden inside it.
Open the message that carried it
Drop the original .eml or .msg into the viewer and see what the message actually contained, including the attachment list.
Open the viewerOr read where the file comes from first.
Where it comes from
winmail.dat is produced by Outlook when a message is sent in
Rich Text Format. Outlook packs the formatting and the real attachments
into a single proprietary wrapper called TNEF — Transport Neutral
Encapsulation Format, a name that is almost exactly wrong, since nothing outside the
Microsoft world can read it.
The sender has no idea it happened. On their screen the message had a spreadsheet attached; on yours it has one unreadable file.
What you can do about it
- Ask the sender to resend as HTML or plain text. Unglamorous, and the only fix that solves it at the source. In Outlook the setting is on the message itself, under Format Text.
- Open the original message instead of the attachment. If you still have the
.emlor.msg, reading the message shows you what it really carried. - Avoid online "winmail.dat openers". They work by uploading your attachment to their server — the attachment is precisely the part you do not want to hand over.
Stopping it happening to you
If people tell you they receive winmail.dat from you, your Outlook is sending in Rich Text. Setting the default message format to HTML for all new messages ends it permanently; the option sits in Outlook's mail composition settings.