I'm not really sure how we are supposed to give feedback to a closed alpha test, but many people here are discussing the business viability of native email clients, so maybe you can talk about the market and what the business prospects are when entering this market.
There's definitely a large subset of the market which expect not to pay for an email client, but there are many email products available now with many paying customers e.g. yesware, streak, sidekick, etc. There are certainly ways of capturing revenue from the email marketing, but I think that Mailbox was too focused particularly on the entire 'Inbox Zero' aspect of the product and never expanded much beyond that.
