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From our friends at pchell.com
What is this winmail.dat file?
Many times you might receive an email with an attachment called winmail.dat. The file is sent from people using Microsoft Outlook as their email program. The file allows Outlook users to send Transport Neutral Encapsulation Format (TNEF) information along with the email so that it retains the look and feel of the document with formatting, fonts, and colors when its received by another Microsoft Outlook user.
Unfortunately, the file is only relevant and used by Microsoft Outlook. Users of Outlook Express, Eudora, Netscape Messenger, and other email programs will receive the attachment called winmail.dat with the email.
The file is attached to the email because the original sender is sending emails in Microsoft Outlook Rich Text Format instead of Plain Text format.
How to Configure Outlook NOT to send Winmail.dat attachments
To Turn off Rich Text sending for messages in Microsoft Outlook
1) Click on Tools
2) Click Options, and then click the Mail Format tab.
3) In the Send in this Message Format list, select Plain Text, and then click OK.
This will set your default sending method to Plain Text, which will lose your special formatting options with fonts, colors, etc. However everyone, no matter what email program they are using, will now be able to receive your email with no problems.
There is more information on this topic in the Microsoft Knowledge Base