Delta Chat is here, now, and has been for a while. It works on Android, Linux, MacOS and is being developed for iOS (limited function beta is available). There is no Windows client yet as far as I know. Delta chat has a simple and concise website which quickly comes to the point and does not try to upsell anything.

This... has a 'modern' website and not much more. The site starts with a page-filling image of a happy woman in an urban environment, she's clearly exited over something the lighted slab in her hand presents her. The thing sorely missing from the site is the download link and the link to the source repository where all those dreams are made true. It does contain one interesting piece of information though:

> COI for Developers

COI is the only ecosystem that provides openness, a strong messaging basis AND overcomes the network effect. With COI you don’t need to develop, host and maintain your own communication servers. Thanks to Delta Chat Core, as a client developer you do not even need to deal with IMAP directly.

Thanks to Delta Chat core... in other words, this is based on Delta Chat. According to one of the project founders they are cooperating with Delta Chat to ensure interoperability [1].

[1] https://fosdem.org/2019/schedule/event/chat_over_imap/ - "...We cooperate with Delta Chat open source project to bundle efforts and ensure interoperability. ..."

Primarily I guess this is an open specification, whereas deltachat is a closed app? sounds like the same underlying principle to me though. Apparently in the future there would be optional extensions on IMAP servers to make things more efficient.

EDIT: looks like delta.chat is open source https://github.com/deltachat/deltachat-desktop