In this case, though, I come to sing the praises not of a hypersonic airborne truck but of the second major version of the Thunderbird e-mail client. For me, it has two new must-have features that fit in really well with the way I work:
You can have replies filed in the same folder as the original message.
There's a folder view that just shows folders with new messages.
I have a lot of message filters active that file incoming mail into per-topic folders. Knowing which I need to look at, and not having to re-file outgoing replies, will save me a lot of time.
The one "gotcha" so far has been that some of the key bindings have been changed, at least on the Mac. In particular Shift-Pretzel-M no longer creates a new message, but instead moves the currently selected message to the last folder you moved something to… hilarity ensues. After some initial cursing along the lines of "where on earth did that message disappear to", obviously.