Frankly, the biggest knock I have against flexible width designs is that they often encourage usability problems. There is a reason newspapers and magazines have long used narrow columns: you can read narrow columns faster and easier because it requires less eye travel. Jason even did a little research on this a few years back to find some documentation on this that we could use during a debate at work.
Mozilla Thunderbird has support for the three column view, but for some reason I haven’t really liked it – I think because I like being able to see the sender, subject, date and status. I don’t use the three pane view though, I open evey email in a new window – even when I use Outlook.
Give reading in narrower columns a try, you might be surprised at the result.