Not the OP, but if he’s still looking, I’d recommend he give MailMate¹ a try. In my opinion, it’s been the best mail client on OS X for a while now.

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¹ — http://freron.com/

I'm still hanging on to mailbox. I'm giving them another iteration cycle or two to fix stuff. So far their product and support has been very underwhelming. I still like their iphone app.