Gil has posted part 1 of a great history of Mac email clients from Microsoft. This part covers the corporate clients. The next part will cover IMN, OE, and Entourage. I often get questions about whether Entourage is Outlook for the Mac. In a lot of ways it is - it's the PIM that comes with Office and it (now) talks to Exchange. But, there's a lot of complex history behind it. Now, I can just point people to Gil's posts :-)