Nylas N1 (Score 1)

I'm also looking for a replacement for Thunderbird. For me, conversation view is a desired feature. Geary was a good option, but it is no longer maintained since Yorba team is ended. Fortunately, I found a new e-mail client that looks promising: Nylas N1. It has conversation view, it's easily configurable and it's open source. Currently it lacks a proper addressbook, but it's still in development. You should take a look: https://nylas.com/N1/