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).