Winmail.dat files 

What are winmail.dat files and why do I recieve them?

Winmail.dat files are a proprietary encoding for email sent by Microsoft's Outlook email clients to non-Outlook email clients.  Specifically the problem is caused by the handling of RTF (Rich Text Formatted) files being sent.  This problem manifests itself as an attachment winmail.dat in the sent email.  The .dat file cannot be opened without the assistance of a specialized utility.

Solutions

Receiver

If you recieve a winmail.dat file, there are a number of utilities that will unpack the attachment for you:

• Linux - Fentun
  
• UNIX - Bob Krzaczek wrote a filter for procmail that fixes this problem.  
• Mac OSX - TNEF's Enough
• Windows - WMDecode

Sender

The sender is the source of the winmail.dat file.  The only way to prevent your Outlook client from sending winmail.dat files is to make it send all emails in plain text format.  RTF or HTML formatting can generate the winmail.dat attachment.  

Preventing the winmail.dat files in Outlook: 

1. Select Tools->Options from the Outlook menu bar.
2. Select the "Mail Format" tab in the dialog window.
3. In the first drop-down list, under the "Message Format" heading, select Plain Text
4. Click the "OK" button.

Note: This problem no longer occurs in Outlook 2007.

Refence: http://support.microsoft.com/kb/q138053/