Please don't mistake this for flame, but instead its intended to be constructive:
As a UI/UX guy, I would say:
1) The homepage sucks. Really. The red color is so bright that it would attract all the user's attention and would make them ignore the important features you have advertised below
Red is used to implement anything important (like call-to-action buttons) and it looks like you are over-using red. Its a very bad sign, especially when the UI is one of the selling points, and especially when you are charging a premium ($20).
2) >A complete replacement for GMail
a) Is going to attract a lot of negative attention,
b) is not ethical
Why? Because it is not a complete replacement for Gmail and never will be.
Why not? Because of various reasons - UI, Features (Some features in Gmail labs aren't available with yours) and finally Price.
3)$20 a month for email is overkill. And there's absolutely no reason why as an end user I should use yours over Gmail, because its free. From what I see, some of the features you claim to provide, which you claim to make your app superior to Gmail fail to impress me. For example, the event scheduling. I'm sure I can do something close with Google's calendar app?
4)If you are targeting Gmail users, then, for what platform? PC? Mac?? Android?? iPhone??? Be clear, right now its hard to digest if this is just a weekend hack or a serious app, because the pitch of your app. isn't clear enough, atleast not on the homepage. (Being brutally honest)
5) There was another guy who tried the same thing - I believe its called Mailapp. It would be good for you, if you could learn from his mistakes, you can find his post here on HN and how much criticism he received despite his UI being significantly better than yours, by many folds. Read what people expect, from the comments here on HN on the mail app, I believe it would help you a lot.
There is a fundamental problem with such apps. Embracing Apple-sque minimal User-Interface design is a good idea only when you know what you're doing and how things should look. There is a reason why important elements like the compose button in Gmail are in red. There is a reason why links are in blue. First try to understand these reasons behind the psychology of colors before you get minimal. Making something/Marketing something minimal doesn't make you smart or superior, infact you will be offending many users because of your poor understanding of UX. Look at the home page, look at the red color for the unread messages. In one sentence - Its highly un-professional, especially considering the fact that you charge me $20 for this.
See, we live in a time where for $50 a year we get an account on a cool photo-sharing site like 500px. Please consider reducing the price if you want to get in more users. For $20 a month, one can run their own VPS (Linode) and run their own E-Mail server..(but not sure why anyone would do that thought, but just saying)
Please do not get offended, I sincerely want you to improve your app. All the best, my friend, wishing you good luck!!