Try to start it directly from Applications folder in Finder. What happens in that case? If it works as you expect it to, remove Mail icon from the dock. Shutdown your computer. Start it up again. Try to launch Mail manually from Applications folder in Finder again. If it still works as expected, add Mail icon back to the dock. Also, when you shutdown uncheck the box that offers you to remember windows running before log out. As an alternative troubleshooting step, log out, and log in as Guest or any other account you might have. I.e. try to launch Mail using a different profile.