This in spite of the fact that I have Background App Refresh off for all apps, and have set Mail to fetch every 15 minutes. In the last 24 hours, charge has dropped from 100% to 30%, and Mail has used 45% of that (showing only 2 minutes on screen and 2 minutes background). My battery health reports 89% max capacity. If the absence of Background App Refresh would have a significant impact on your app’s user experience, you can check the background Refresh Status property of UIApplication to determine whether the feature is available. Users can disable Background App Refresh for your app or for all apps in Settings.

To quit (close) a Mac app normally, choose Quit from the app's menu in the menu bar, or press Command (⌘)-Q. If the app doesn't quit, follow these steps to force the app to quit.

How to force an app to quit

Press these three keys together: Option, Command, and Esc (Escape). Or choose Force Quit from the Apple menu  in the upper-left corner of your screen. (This is similar to pressing Control-Alt-Delete on a PC.)

Then select the app in the Force Quit window and click Force Quit.

You can even force the Finder to quit, if it stops responding: Select Finder in the Force Quit window, then click Relaunch.

Learn more

Mac How To Delete App

How to force an app to close on your iPhone, iPad, or iPod touch.