The cloud is always on and online? I don't mean to be argumentative, but your cloud apparently couldn't handle the load. Downloading an email client shouldn't require a "hit by a bus" analysis of the app's creators.

An app can store metadata locally; no need to have that on your servers. No doubt you can do some interesting things by having a server-side infrastructure, but I'm concerned about the security implications. If your server gets hacked, an attacker would have access to all of my email. Not to mention that you would have access to all of my email and why should I trust you?