I found Mail.app to be very quick to use, especially with Mail Act-On which gave me tons of filtering and best of all, hotkeys that made moving, archiving and applying any number of rules super easy.

The price has gone up a fair bit since I had to move away from Mac, so I don't know if the value proposition is still there, but I know it saved me hours of time...

Thunderbird is the closest to productive I've gotten on Windows since, but I want those hotkeys!