Thunderbird has consistently been the least bad mail client. I've tested just about every open source client and even a few proprietary ones but nothing fits my needs. A few are close but missing some show-stoppers (like the ability to pull up full threads from a single message in my inbox).

After seeing this I donated again. $100 this time. I recommend everyone else consider how important the software they use is to them. For me Thunderbird is essential and used daily so <$10 a year is a no-brainier.