This is the most overhyped app I've ever seen. I do not think it looks very innovative or useful, all of your e-mail gets routed through their server, and it only works with gmail. Yet, every time Mailbox's developers sneeze the tech bloggers are all over it, tweeting rapturous praise.
And this "reservation system" seems very suspicious. Give them your phone number? I don't think so! This is a piece of software. Spin up more servers if you have too much demand. If they can't quickly respond to user load, their architecture was badly designed. The tech press should be slamming them for this, but again it's all fairy dust and rainbows.