I'm not sure what operating system you have, but I use Evolution on Linux. There's also a fork of Thunderbird called Betterbird that I used for a while, but the UI was a little clunky on Linux.
It'll be on windows machines. I'll check out betterbird, perhaps it can be better on windows. Would u say the ui is a significant improvement over thunderbird despite the clunkiness??
It's very similar. The primary difference is the amount of bug fixes that have gone into Betterbird versus Thunderbird. It's pretty much the exact same UI though.