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).
Haiku's mail system may be of interest to you -- the default UI is of course a GUI, but as the mails are stored as individual files, and their headers in extended attributes, you can use "query mail:from=john", etc. to filter mails, "mail" to send them, etc.
Actually, I was a big fan of MH. But it's quite old and full of gotchas. It doesn't have decent search functions either. Also I'd like to have something written in a modern scripting language (Python, JS, etc), as I don't want to worry about buffer overflow.