When someone uses Microsoft Outlook to send messages through the Internet with attachments and they use the Microsoft Outlook Rich Text format, some recipients report that the message includes an additional file called the Winmail.dat file. The Winmail.dat file is usually very small, but you cannot open it in the message. The original message attachment is not always separate from the Winmail.dat file attachment, and may be included in the Winmail.dat file attachment.

This problem occurs because the Winmail.dat file is used to preserve formatting that the sending client includes in the message, but the receiving client does not recognize the Winmail.dat file. In Outlook, the Winmail.dat file includes Rich Text Formatting (RTF) instructions. This type of formatting is used with the Microsoft Outlook Rich Text format and when you use Microsoft Word as your e-mail editor. Only other Outlook users can read the attachments.

To resolve this problem the person sending the attachments with Outlook needs to do the following:

1. On the Tools menu, click Options and then click Mail Format. 
2.  In Compose in this message format, click to select Plain Text, and then click OK.