It is already usable as an alternative Gmail interface with a set of unique features:

  - internal lightweight tabs
  - linking few threads together
  - composing emails with Markdown

Public demo: http://mail.pusto.org/

Short demo video: https://vimeo.com/145416826

Github page: https://github.com/naspeh/mailur/

The first intro: https://pusto.org/mailur/intro/

Backend. Python3. Main JSON-RPC server and WebSocket server for push notifications (with help of Werkzeug, psycopg2, aiohttp, lxml).

Frontend. ES6. Single-page application based on vuejs. Also it used less as CSS preprocessor and browserify for bundling up all dependencies.

For clarification

Mailur doesn't have support arbitrary IMAP server yet, but it has basis for implementing this, just need more time for development and testing. It has bidirectional synchronization for Gmail through IMAP.

Maybe it would be more accurate to say this is a Roundcube replacement with Gmail inspiration. But for me it is Gmail replacement, because I have used it over 8 years and want to bring Gmail experience to open source.