Q: What is the WINMAIL.DAT attachment file that I receive from people in my email - I am unable to open it with any program on my computer.
A: The situation causing this is that people are using several different email client programs to receive, read, and send email. The most commonly used email client programs is Microsoft Outlook and Netscape (specifically the Messenger component), with a small minority of techno-geeks using Eudora. Unfortunately, Outlook does not "play nice" with the other email programs all the time which causes problems, not for the sender of the email, but the recipient, particularly when actual files are attached to messages.
These rich-text email programs send the formatting information which your computer/email program can't read, so it creates the winmail.dat file to contain the rich-text formatting content, instead of just throwing it away.
The Solution: If you receive an email with a winmail.dat attachment, just ignore the attachment. That is, unless you would rather purchase and use a "rich-text" email program to enable you to read these rich-text emails people are sending you.
Outlook 2003 email program - a few of examples showing rich-text email