I already said Thunderbird rocks. And little by little TB keeps reminding it to me every day.
E-mail wrapping is something you can have tons of fun with. Basically some RFC says that plaintext e-mail should be wrapped, yet many mailers don't do it when you're sending your mail. And then mailers behave differently about received mail -- some wrap it and some don't. So all in all it's a big mess where similarly to HTML, most of the world has applied "creative" approach to standards.
So let's say you receive an unwrapped e-mail. Thunderbird displays it wrapped to display width, yet if you try to respond to it, the rows are NOT wrapped and you may have 300-column rows where the whole paragraph is contiguous. And it's just not nice. Yet it's correct.
You may want to fix this error and have your response and the quotes in it rewrapped nicely. So what does Thunderbird have? A handy "Rewrap" function which neatly rewraps your quote. Simply brilliant for hardcore users.