People who use email clients other than Microsoft Outlook may find that they receive winmail.dat files as attachments. Those attachments are of MIME type "application/ms-tnef". The attachments likely originate from senders using Microsoft Outlook. If the sender has Microsoft Outlook configured to compose messages in "Rich Text" format, then Outlook will send attachments in Transport Neutral Encapsulation Format (TNEF) format, which may result in the recipient of a non-Outlook email client seeing attachments named winmail.dat.

You could ask the Outlook sender to reconfigure Outlook to send in another format. The sender really should be willing to do so, unless they are a Microsoft shareholder hoping to help Microsoft lock users into its products, since the change needed in Outlook is fairly trivial to make. In Outlook 2000, the user need only open Outlook and click on Tools, Options, then click on the Mail Format tab in the Options window and change the setting for "Compose in this message format" from "Rich Text" to "HTML" and click on OK (I'm not sure if the user then needs to restart Outlook).

There are also options for dealing with the issue at the recipient end. If the recipient is a Unix or Linux user, TNEF provides a way to extract files from the TNEF archive file, such as winmail.dat. It is a process akin to unzipping a zip file to view the files within it. TNEF is also available for Windows, if Cygwin is used.

TNEF can be obtained from To use it on a Unix or Linux system, you can use the following procedure (for this example, I'm assuming that version 1.3.2 of TNEF is being used):

  1. Unzip the file with gunzip tnef-1.3.2.tar.gz.
  2. Untar the resultant tar file with tar -xvf tnef-1.3.2.
  3. Change the working directory to the newly created tnef directory with tnef-1.3.2.
  4. Run the configure script with ./configure.
  5. Compile the software for your system with make.
  6. Install the software with make install.
  7. You should then be able to view the help information for the program with tnef -h (see TNEF Help for TNEF options). If tnef isn't in your path look in /usr/local/bin/tnef.

You can list the files contained in the winmail.dat file without extracting them with tnef -t winmail.dat (I think of the "-t" as standing for "test"). To extract the files type tnef winmail.dat. You can then view the extracted files with a suitable viewer, i.e. Adobe Acrobat for PDF files.

If you don't want to install the software on your system, Balázs Bárány provides an online file extractor at on his MS-TNEF degenerator webpage.

Note: TNEF is mainly tested and used on GNU/Linux and CYGWIN systems, but I've used TNEF on a Solaris 2.7 system without problems.


  1. What is the Winmail.dat file attached to my incoming emails?
  2. Dealing with winmail.dat and unreadable email attachments
  3. Description of Transport Neutral Encapsulation Format (TNEF) in Outlook 2000
  4. MS-TNEF degenerator

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Requirements: All POSIX (Linux/BSD/UNIX-like OSes), OS Portable (Source code to work with many OS platforms). Cygwin needed for use on Windows systems.
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Created: June 9, 2007