Email users sometimes find that they receive email messages with a strange file attached, called winmail.dat. This file is an attachment generated by Microsoft Outlook and is intended to be decoded on the recievers end using Outlook, however the reciever may not be using Outlook.
When you open the attachment you find that the contents are garbage.
There are two solutions:
1. Ask the sender to send the email again but using plain text format. winmail.dat is only generated when the sender uses Outlook and sends emails in HTML, or Rich Text format.
2. Visit http://www.biblet.com and download the WMDecode program. This will allow you to decode the winmail.dat and extract any usefull attachments from it.