Good points. Thank you. About searching the email inbox: Upon first signing on, all emails (and filenames) in the inbox will be indexed and cached to MySQL and we're also working on providing an option to use WebSQL to cache this locally.
To be honest, I think email search is something even Google hasn't succeeded yet. The way it's introduced on your homepage makes me think you have succeeded...but it will definitely be HARD to scale if your service becomes popular.