What is the crowd's opinion of Kube?

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thunderbird - one of the few constants in my life. I use it for decades, literally! :D but i always fear that no one will improve it, because of XUL.

and then i look at kube (https://kube-project.com/) but that has not that many developers (but with C++, QT and QML also pretty plattformindependent), and they want to do another lookandfeel and i return to thunderbird.

Kube? It did look promising, though the development wasn't very active for quite a while now unfortunately, not sure this will ever end up being a complete Kmail replacement.