Which Alternative to Apple Mail Do You Prefer (if any)?

Last week, I made the mistake of upgrading to the new Mavericks operating system on my MacBook Air. I immediately began to experience problems with Apple Mail and my Gmail accounts. (The problems have been documented in numerous places, including this article on TUAW.) I am now testing alternative email clients. Which email program do you use? Why do you love it—or hate it? Tell me in the comments below.

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