https://github.com/kanru/notmuch https://github.com/kanru/notmuch This has thread views. It works very well. If you look at the headers of an email you can see

https://github.com/kanru/notmuch https://github.com/kanru/notmuch This has thread views. It works very well. If you look at the headers of an email you can see: In-Reply-To: and References I'm not sure that this is what it is using, but it is a possibility.