This issue started for me shortly after the Yosemite upgrade. I'm on a 2011 MacBook Pro, 13" When opening Mail, the window will come up with only senders and subject lines for existing emails (left-side pane - the two-line email preview is missing). In addition, the preview pane will show nothing, even if I click on an existing email. New emails are NOT received, and any emails composed at this time are NOT sent. Waiting does nothing - the app stays in this useless state. Closing the app requires a Force Quit. At first, re-opening the app would solve all the problems, and it would behave normally. Now, more often than not, it requires TWO Force Quit/restarts to get the app to load and function properly. What is the deal here? This is getting really frustrating...please help!