I tried for a long time to work with Thunderbird, but what kept bothering me was that I couldn’t simply define keyboard shortcuts. In the end, I landed on AERC and created my own extreme Vim-style keyboard configuration (the idea is to look at the list of mails like looking at a buffer in vim) for it. I’ve never been this fast when it comes to email.

https://aerc-mail.org/ https://github.com/rafo/aerc-vim