I installed Thunderbird 3 on the N900, using Easy Debian (it is called "IceDove" in Debian). It can be found in the "Experimental" repository. It runs reasonably quickly, but I didn't get the UI set up very nicely, and so it is quite cramped and difficult to use.

Hey, I just discovered that it is installed in the latest image available for Easy Debian. So you can try it out yourself if you're a hacker type who can download and extract the image file manually