So I was trying to check my email and all of a sudden Mail.app was not responding. I had to Force Quit the app because all I was getting was the spinning ball.
I tried again and the same thing happened.
So after trying a few different things I decided to disable all my network connections and it finally opened. I just turned Airport back on and Mail.app was working fine again.
By the way, I have Mac OS X 10.4.11.