Ditto. I use ThunderBird (well, IceDove these days), and I kinda love it, but I'm well aware that e-mail clients have been relegated to oddities. And I do have my complaints about ThunderBird, by the way. I'm tempted to browse the whole of its source in order to track down the line of code that makes it open any message at random when you open a directory, then file a patch to eliminate it.
E-mail clients could be so much more than they ever were, though. And without breaking any protocol. But e-mail itself has lost a lot (not all) of its relevance because of spam, despite there existing protocols specifically aimed at killing the latter. Inconvenient for base users, yeah, but that's where clients could have provided facilitating tools. That's why fenced environments gained traction, again.