Microsoft's Email Client
*** see also www.slipstick.com which has everything you could ever want to know about Outlook
Outlook is a powerful, yet very intuitive interface for email. When it first came out, within months it had eclipsed the well-known Eudora, Pegasus, and CC Mail. The main reason for it's instant was that it had the first Windows style GUI (Graphical User Interface).
The Outlook GUI had a preview pane that sat below the list of messages, which could show simple text, or HTML messages. In addition, all attachments were shown as icons (shortcuts to the attachments). At that time, no other package offered these features.