Do you mean a self-hosted webmail app? Or a native/multi-platform native email client? Personally I think Gmail UI is meh - but I no longer use email that much

Do you mean a self-hosted webmail app? Or a native/multi-platform native email client? Personally I think Gmail UI is meh - but I no longer use email that much - so terrible UI/ux and no proper quoting/threading support isn't all that problematic.