The (minor) bad first: your landing page is utterly broken without JavaScript (I use NoScript, and while I whitelist most sites, a broken page before setting that still leaves a bad taste if there's no reason for it to break), and I can't see anything you do on it that couldn't at least gracefully fall back. At the very least, if you wish to require JavaScript, you might want to say upfront there that it'll be required.

As for the good: I'm liking a lot of where you say you want to go with nvlope, and I can't say I particularly enjoy IMAP, from working with it, so your planned API is more than a few steps up. :)

Can't wait to see how things develop!