Use the instructions below to remove an Exchange account in Apple Mail 10.5 and 10.10.
Exit Apple Mail if it is currently open
Click on the Apple in the upper left corner of the screen.
Click on System Preferences.
Click on Internet Accounts.
On the left side of the window that opens click on your Exchange Email Account.
Click on the minus sign (-) in the lower left hand corner. You have successfully deleted your Exchange account.