Why Do I Receive Winmail Dat Files ?
Winmail dat files are generally received when your mail program is unable to properly decode the email being sent to you.
This can happen for a number of reasons, but the most common is due to different email clients. For example if an email is composed and sent from Microsoft Outlook it allows extra information called Transport Encapsulation Format or TNEF for short to be sent with the email, this extra information contains information such as fonts, colors and formatting, and if this email is received by someone using outlook then it retains the original look and feel. Because TNEF is only relevant to users of Outlook, other email programs such as Netscape or Outlook Express will also receive a dat file attachment. To get around these problems, make sure the email sender turns off RTF or rich text format. To do this from Outlook select Tools > Options > Mail Format tab, and change Compose in this message format to Plain Text 
Using plain text you’ll lose options such as colors, font types etc, but no matter which email program opens the email it will be fine. It will also take up far less space than email composed using rich text or HTML If you really want to read the contents of the winmail.dat file then you have a the following options. - Install full blown Outlook, a trial version can be downloaded from Microsoft
- Search the web for an application such as WmDecode which will allow the attached dat file to be opened.
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