I understand why you put in the drawbacks section, I know that kind of anti-marketing can work. But it's a little too honest - nobody wants to read that their emails might end up as junk mail. Maybe be a bit more vague. Or if you're not significantly worse than your competitors on that front it's a bit misleading if you're disclaiming it as a drawback.

Also, when you open up a link in the menu as a new tab (via middle mouse button) the current tab becomes unusable due to the loading icon you overlay. Very annoying!

And finally - it'd be good to have a demo of the web client without having to register. Because that's essentially what most people will be paying for since email plumbing is pretty standard across providers.