What decent email client would you recommend (at least better than Thunderbird)?

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http://askubuntu.com – I've used Thunderbird for years. I keep a huge number of emails. I move them to folders to organize or archive them, but I don't delete anything so I have hundreds of thousands of messages. I like the way TB is conceived, and the way it works as long as the volume of data is small. But it just doesn't scale. It has a lot of ridiculous design flaws such that, for example, any time consuming operation blocks the whole UI completely (and you don't even know for how long) as if everything was implemented in a single monolythic all-tasks-are-blocking way. I'm tired of it. So what is the alternat (HowTos)