.....

• allow me to use my own domain, not just for the website but also for email
• allow rich text emails to be created from the web interface
• speed up tremendously

Then it would be the perfect tool and really be the "Exchange for the rest of us", as advertised. But for now I stick to Google Apps and Mail for a while, they offer it (for free even) and with help of Nuevasync synchronization with the iPhone is an easy task.