KB07011601 Problems with Winmail.datUsers sometimes receive email messages with an unexpected file attached, called winmail.dat When they attempt to open this file, either it can't be opened at all, or it contains unreadable data.
They also may find that attached documents or images are changed, for example, from the expected image.jpg to image.dat - and are unreadable in their email client.
This is not a Mailtraq issue at all, but is to do with Microsoft Outlook's use of an unusual format called Transport Neutral Encapsulation Format. Unfortunately, Outlook is the only email client program that uses TNEF. The problem arises when messages that have been sent from Outlook are received in another email client.
It means that messages sent from Outlook can be rendered inaccessible to other email clients.
Microsoft provide this KB article on configuring users' Outlooks to solve the problem.
There are also a number of accessible articles to be found here that explain how to deal with this issue. | 
