I was confused by your answer regarding winmail.dat. The file cannot be opened traditionally, at least to obtain what the receiver believes to be valuable contents. Rather, it's used internally by ...

So instead of seeing the "rich text" formatting of the original message, you get a plain-text copy, plus a mysterious "winmail.dat" attached file.

We keep getting e-mail attachments in winmail.dat files, and can't open them. What's going on? Is there a utility that can extract the attachments from these?