Things I like:

- Delivery tracking. This is the single most exciting feature that I found in your screenshots (welovemail.com). Mass mailing solutions and Transactional email services have had this feature for ages, and it's about time regular email got it, too. No more need to parse the incoherent ramblings of MAILER-DAEMON, hooray!

- Three-column view. (Or was it four-column?) I'm not particularly fond of 16:9 widescreens, but I might as well use all the space since I can't seem to buy anything else anyway.

- Managing multiple accounts.

- "The only way we can build a product that we can go and use without any stress today is to make the best out of what’s already available." -- Absolutely agreed. Email is email. It's not a todo list, it's not an instant messaging protocol. I'm glad you're taking this seriously.

- "Get a receipt from a local store directly to your inbox formatted the way for use in an accounting app." -- Wonderful. Don't stop with a pretty app, please go ahead and write an RFC that defines the standard markup for electronic invoices, receipts, to-do list items, etc. Then we'll be able to use SMTP, HTTP, or any other protocol to ship them around.

Things I'm not sure I'm going to like, although it seems early to make a judgment:

- "How simple a life could be if every citizen would own a mail account allowing them to vote in any election from anywhere in our solar system." -- What does email have to do with voting? My email account is not my identity, no matter how hard Mozilla Persona et al. tries to shoehorn identities into email addresses. It's just an address that I happen to be occupying at the moment, and that I can leave behind whenever I want. So please don't try to sell your product as a candidate for universal adoption from the first day. You'll be disappointed.

- Managing multiple accounts. Although I also mentioned this in "things I like", one of the reasons I keep multiple accounts is to put firewalls between them. For example, my primary email account is never accessed from a mobile device, I use a "proxy" account for that. The password recovery address is yet another account at a different provider. If one of my accounts get compromised, I don't want the attacker to gain access to any other account. How is your app going to manage user credentials and flow of information to/from your servers to minimize vulnerabilities?

Some questions/suggestions about the UI mockup:

- Social media integration, from Facebook/Twitter in the sidebar to nicely cropped photos for all your correspondents. Please tell me this can be turned off. When FastMail.fm rolled out its new interface a few months ago, there was a significant outcry from privacy-sensitive users who discovered that the webapp was pulling in everyone's Gravatar.

- Where's the metadata, like those ugly CC/BCC/Reply-To lines, "display source" links, and the message size? If you leave them out, you'll be lacking some of the features that email pros can't live without, and therefore earn the criticism that you're trying to replace email with something less. On the other hand, if you put them back in the UI, it won't be as neat as your screenshots currently are.

- Can I compose full-screen, or do I have to live with 100px of darkened, unusable space around your cute pop-up whenever I'm trying to write something?

- Is this a hosted web service? iOS/Android app? OSX/Windows program? All of the above?

/edited