What I want is a GMail client on premise. An open source clone of the GMail Android app or webapp running in Electro or Servo. GMail has a perfect conversation-style UI. Even today Outlook, Windows Mail, Thunderbird have no equivalent conversation view (just hacked together additions that don't work). Also GMail introduced the concept of tags instead of lame folders (an email can be tagged with several tags). Also GMail syncs notes and calender with CalDAV and other open protocols. It works fine out of the box with iOS. Even the notes app syncs data to GMail IMAP out of the box.

The old Opera 12 browser featured such a similar HTML based email client. Vivaldi promised such a email client, but no news for years.

Thunderbird is like Outlook Express aka Windows Mail (Windows Vista/7/8) (both deprecated), Outlook's little brother.