I like Sylpheed ... especially because of its superfast lightness, and its ability to thread messages (like the bat!).
Sylpheed-claws is available for windows too ... its an extended version of sylpheed, supporting HTML, and some other extra features.
BTW, thunderbird is not light
... I have tried TheBat, Eudora, OE, Sylpheed and many others, and thunderbird was slower than them all.
Mozilla's inbuilt mail clien is a little better though ...