Which Free Mac Email Program Do You Use?

The peel inside, sweet and bitter: marmalade is every breakfast's delight. Typical marmalade is made from Seville oranges, which is a bit unfortunate because the original, Portuguese, "marmelada" is, of course, a marmelo — quince — mush.
From a quince turned orange, now, to an apple turned computer: what's that sweet email program delight inside yours?
›› Do you use the email program that came with your Mac, or do you have a different favorite free email app? Share your pick here, and find out what email programs others prefer.