Apple mail
Apple mail window fills the screen and cannot be moved. Navigation bar at the top of the screen is covered. No way to click an icon at the top. This is a new problem. I have a MacBook Pro, running El Capitan 10.11.6
Mail window may be in the Full Screen mode.
Press the "esc" key on the keyboard
or
move the mouse pointer to the very top of the screen,
click the green navigational button when the toolbar drops down.