I'm confused. Is this only for Mac (it looks like it from the screenshots)? I find it a terrible business move to have a start-up release their product only for a platform that is much less than 10% of the world's personal computing marketshare.
It looks nice, but I think you have shot yourself in the foot for trying to release a Mac app first. Windows doesn't have any good email application that is still developed anymore. Pretty much the choices are Outlook and Thunderbird (the latter of which was just announced would have no more feature developing). People are looking for the next email client to flock to...
It looks nice, but I would suggest adding some color. Having just plain black text (with some green) on a white background is kind of boring and doesn't get the hype of your startup across. This landing page doesn't make me "excited" to put in my email.
Try adding a cool background image and/or more color.