Winmail.dat – Plain Text Not Working

I have a user in Outlook 2000 on an NT sending a Word attachment to a user on a Macintosh. The attachments are being turned into Winmail.dat. I had my user resend the message in plain text, and the recipient was still unable to open the attachment. Does anyone know the best way to send an attachment from a Windows environment so that it is readable in a Mac environment? Would zipping make a difference?