As another commenter pointed out, CLI/TUI isn't that hard. In many cases it's easier than GUI ones. But I have a different purpose. I can configure different pieces (imap for incoming, smtp for outgoing, notmuch & afew for tagging and search, etc) and use it uniformly from a variety of different programs including git and emacs. Not very simple, I must admit. But it's a personal choice. It works very well for my use cases, including realtime full mail backup and offline use.