Dock does not show up in Mail Application

Welcome to Apple Support Communities

Do you see the menu bar when you open Mail? If not, you have opened the app in full-screen mode. Just move your cursor to the bottom of the screen to show Dock, or move your cursor to the top right of the screen and press the blue rectangle to restore this app in a window

You are welcome

Its been 4 years but I have the same issue. Only I don't see a blue rectangle anywhere on the screen

Hold down the option/alt key and click the green button at the top right.