What mail app do you use on your mac? I personally use the built in mail app. But what do you use?
Sparrow for mac. Best mail client I've used.
http://sparrowapp.com/
There's a mac version and an iPhone version. I use both. As a bonus, if you use Gmail, there's excellent Gmail integration. The full mac version costs $10 via the app store, but you can use the ad support version for free. I spent the money and I haven't looked back.
Gmail with Google Chrome...I've never seen a good reason to use a dedicated email client, but that's just me.