I'm a bit wary of this move.
I have been using K-9 mail for over a decade. Last year sometime cketti pushed out a new update that changed the main interface of the client removing one of its most useful features, an overview screen of all your multiple accounts.
I've been using it for about that long myself, and I'm afraid that I don't recognize that complaint. There is still the Unified Inbox view, and all of the accounts are visible and select-able in the menu. What "overview screen". Perhaps I should just be happy that I don't know what I'm missing.
I'm less certain about Thunderbird. When it was part of Mozilla I remember it being clunky and ugly, and made me use Sylpheed instead, until I switched to macOS where I've been (mostly) happy with AppleMail.
