Mail app 7.0: what does "Check for new messages" "Automatically" actually do?
According to Dan Frakes @ macworld.com:


  In the General screen of Mail’s Preferences window, you’ll find a new option in the ‘Check for new messages’ pop-up menu called Automatically. This option essentially allows Mail to support push-style message retrieval—you receive new messages immediately, rather than at the next scheduled check.