What is winmail.dat?

You've received an email message that contains a winmail.dat attachment. This issue may occur if all of the following conditions are true:

• The email message is sent to you by someone using Microsoft Outlook. 
• The format of the message is Rich Text format (RTF).

Send the following link to the person sending you the email and ask them to send it as HTML format or plain text ....

Micorsofts Knowledge Base for Winmail.dat