Mozilla seems to fuck up everywhere. Thunderbird used to be great, too, but again since version 3, it got so slow that even a snail seemed like a 400 HP race car.
Since Thunderbird kept one of my CPUs busy for minutes while downloading new messages, possibly because of the hundreds of thousands of mails I have in my mail accounts, before it finally reacted to user input again, I decided it was time to look for something better.
I tried out Sylpheed and I’ve got to say: It’s very good! Not only does it handle the amount of mails I have with ease, but it also has a much cleaner look, while having the same functionality as Thunderbird (at least the parts that I need).