Seeing that you require an email address, the first thing I looked for was your privacy policy. Unfortunately, the lightbox screen doesn't scroll in my browser (Opera 10.10) so I couldn't skim through all of it.
I like your interface, very clean. How do you plan to monetize/keep-it-going?
For now it will run like this, no ads, we mainly use it to test new stuff that we might use in the future for other projects.
For example the smtp server is custom to make sure it can handle huge amounts of mail (to learn some more about writing high performance servers). Also the realtime "new message" notification system is written in node.js which we are now using for different projects because it works so well.