As an initial concept it's interesting but not very useful for most people in it's current state as it ultimately requires me to spend more time in my inbox. I think the real potential is in your "Coming Soon" feature, where it will hopefully be able to automatically figure out the best subject lines for me and save me time in the long run. Ultimately, it'd be great if it could decipher whether or not the email can be answered with a simple answer (yes/no, a time/date, etc), and provide the buttons to respond to it without ever actually opening up the email itself.

As far as the site itself goes - it has a clean, simple to understand interface lacks anything sexy to sell the idea. The animation is a great idea, but just seems dry and lacks context. Looking at the "Using Signal" page (https://trysignal.com/using-signal/), I'd say use a screenshot similar to the one there (albeit with the full inbox visible), and show an animation of the email being edited in real time rather than the side by side comparison. That would be a much clearer demonstration and immediately provide the context as to what's happening. In all reality you could just integrate the "Using Signal" page into the main index for the moment as it doesn't have enough information to really constitute a secondary page.