How to Open a winmail.dat File
winmail.dat is not a document. It is an entire email — body, formatting and every attachment — packed into one file by Microsoft Outlook, and only Outlook unpacks it automatically.
.dat
Open it on this site Why it will not open
- The file has no useful extension because it is a container, not a document. Renaming it will not help; there is nothing inside called "the real file" until it is decoded.
- It is produced by the sender, not by you. Outlook creates it when a message is composed in Rich Text Format, so no setting on your machine can prevent the next one.
- The format is TNEF, a Microsoft-proprietary encoding that no industry standard supports. Mail clients other than Outlook have no way to read it and simply hand you the raw blob.
How to open it
In your browser, without installing anything
- 1 Open the winmail.dat viewer on this site.
- 2 Drop the file in. It is decoded in your browser — nothing is uploaded, which matters because a winmail.dat is by definition somebody else's email.
- 3 The subject, sender and message body appear, with every embedded attachment listed by its real filename.
- 4 Download attachments individually, or all of them as a ZIP.
Ask the sender to resend
- 1 This is the only fix that stops it recurring.
- 2 Ask them to set the message format to HTML or Plain Text instead of Rich Text — in Outlook that is Format Text > HTML in the compose window.
- 3 For a permanent fix on their side: File > Options > Mail > "Compose messages in this format" set to HTML.
It may arrive with a different name
Depending on the mail system in between, the same file can appear as win.dat, windat, ATT00001.dat or a bare .dat. They are all TNEF and all open the same way.
Why some tools mangle Japanese filenames
An attachment carries its name in several places inside the container. The oldest field is code-page dependent and often truncated to 8.3, so a tool that reads only that one returns names like 8E9095948E.PDF. Reading the Unicode long filename first is what keeps them intact.