Winmail.dat is a curse to web hosts.  It is a file generated by Outlook, most often when an exchange server is not properly configured.

There is a quick fix.  Ask the sender to send you messages in plain text format.  Exchange Server and Outlook are often set up to use RTF ( Rich Text Format ) assuming as the makers of the software do, that the whole world uses their system only.

By sending mail as plain text the winmail.dat file is not sent.  Below are some links to Microsoft support pages on how to fix this problem.  You might want to get the person sending you the winmail.dat files to point their IT people to them.

http://support.microsoft.com/kb/138053 How to prevent winmail.dat from being sent to internet users

http://support.microsoft.com/kb/278061 Winmail.dat attachments are included in received e-mail messages in Outlook