OP here. I've used Exchange, and I'd agree that it does solve the same problem. Gmail just makes a better example, not least because I prefer it.<p>Exchange also has the flaw that you can only do most of the clever stuff using Outlook. The web version of the front-end is passable, but it's clearly a fall-back rather than the primary interface - I much prefer GMail's attitude - everyone has a web browser. If you can build a better client than a web page then go for it, but the web interface sets a high standard.