We thought of that a lot. But like our other baby, http://id4me.org - where we chose use use DNS and OpenID and mesh it into a federated identity protocol - email infrastructure is simply there, it's federated, robust, run by millions of servers, large and small, tested, filtered, pounded, drowned, abused you-name-it. That buys some extra overhead.
And yes, we want to open the chat ecosystem to many many cool clients made by you all for every taste and use case. Beyond the obvious WhatsAppySnappyChattySlacky varieties, if you think it through, you can even envision Twitter and Facebook-like clients. Sounds crazy but we cannot imagine why not. Plus normal Email clients can be seriously pimped.
Current thinking is that the IMAP servers connect directly, but via SMPT for message transfer, but without the store-and-forward, once they are trusted via a token exchange. May be blocked though in server deployments (ports).
Chat over SMTP/IMAP/MIME just doesn't sound right, or does it? We also thought of Chat over Email, or Chat over Mail, But Chat over IMAP gave us the COI (fish), which we love. Decision made :)
Chat messages will land in a separate folder for the servers that support COI. For the others they will be collated into normal mail messages with a small delay, but yes, it may end up being many.
We have about 7bn total unique active (3 months) accounts in the world, 50-60% is Gmail, Outlook.com, Yahoo and a few more, 40-50% are real IMAP servers, of which 76% use Dovecot. Very large deployments include companies like Comcast, Deutsche Telekom, 1&1, LGI (Ziggo, Virgin Media, UPC) with 10m+ user deployments, and yes, a veeery long tail of millions of small servers and anything in between. But COI will also play fine with GMail and friends.
