We seem to be experiencing a renaissance of TUI software, particularly email clients[1] and I couldn’t be happier.

Email and TUI are such a natural fit. Make it work with your favorite editor and most nerds will switch.

[1] https://aerc-mail.org/

I, for one, want a pure command line based email client, not just a TUI client, that is well integrated with bash/zsh in an everyday use, e.g.

    $ inbox
    (shows the first 10 emails in the inbox)
    $ search from:john
    (shows emails from john with numbers)
    $ reply 4
    (opens up vi to edit a reply to the email #4)
    ...
I've been dreaming a client like this for decades, and actually started developing it, but the development is stagnating for now (due to the spec changes and other priorities).
You might find mblaze (https://github.com/leahneukirchen/mblaze) interesting; it's not too far off from what you describe.