Threaded e-mail is not exactly a new concept. . . it's been supported since the early by numerous mail clients, and nearly every mail client in exsitence, other than Outlook, has supported it for years now.
This is without even getting started on the horrible habits that Outlook has pushed on to people such as top posting and quoting an entire e-mail (and all subsequent e-mails) in replies.
The more I'm forced to use Outlook at work, the more I realize that it is a truly crappy program.