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The problem

If you are not using Microsoft Outlook, you maybe know this problem: You get an email from somebody using Outlook with an attachment called winmail.dat. If you would use Microsoft Outlook you would not see the attachment but the real attachment(s).
This problem appears when somebody sends e-mails in RTF (Rich Text Format) with Microsoft Outlook. If the receiver uses Thunderbird, Mozilla, Netscape, Opera, Eudora etc. to view emails, he cannot see the contents of the winmail.dat.

How to tell Outlook not to send the winmail.dat

Have a look at the Microsoft Knowledgebase to prevent Outlook from sending that file: Microsoft Knowledge Base.

How to extract the winmail.dat contents

This small tool KM.winmail enables you now for opening, decoding, viewing, converting and extracting the contents of the winmail.dat file.

This tool is freeware and licensed under the GPL as it uses the libytnef.

Supported Operating Systems
W9x, NT, 2000, XP.

1) Download the viewer version 1.1 here
2) Unpack it to a location of your choice, e.g. c:/programs/winmailviewer/
3) When double clicking the winmail.dat attachement in your browser, enter the location of the unpacked kmwinmail.exe

From now on, the winmailviewer will be opened for your Outlook attachments files.

After double clicking the attachment in your mailer, the viewer opens under the mouse cursor:
winmail.dat viewer
Select the file you are interested in with your left mouse button. Now you can choose the actions with the buttons:

"Open" opens the file with the registered application.
"Save" saves the choosen file. A save-as dialog comes up.
"Save All" saves all files in the archive. A save-as dialog comes up.

To open the file directly without using the menu just double click the file in the viewer.

If you have any feedback for me, send me an e-mail to .
You can download the source code here.

Version history

      - Initial revision. Can open winmail.dat files with attachments.

      - Resolved error with paths that are enclosed by apostrophs ("). This
        is done automatically by windows when a path with spaces is passed.
      - Upgraded to libytnef 1.5.
      - Show message if the winmail.dat is a vCard, vCalendar or vTask.

Other tools you could use to view the winmail.dat

Web Service to decode the archive online
TNEF's Enough for Mac OS X

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