You, sir, have obviously never heard of nmh, or its predecessor MH. It's a suite of command line tools for email, and in 20 years of reading email, I've found nothing that yet comes close to the power and capabilities of reading and processing email with nmh. Being able to search for email from multiple folders using regexp and shell scripting just isn't as easy as on any GUI MUA I've ever seen, and because it is just a bunch of single-task binaries, it's very easy to build a GUI or even web-based front-end for it. The only thing that got me to switch from using it as my primary email was the gmail interface, and labels vs. folders. If not for that, i'd still be using it daily, and not cuz of the cool/smug factor, but simply because it works, and works well.