macbook – The Mail application continues to appear in the background in Mac OS Mojave

When I work on my macbook pro, the main window of the Mail application keeps getting displayed in the background. It even appears when I watch movies by interrupting the full screen mode. It's so frustrating that I can not find a solution. Googled and accessed several discussion forums, no chance, suggestions that I found very old and outdated.
Has anyone encountered similar problems or does he know a solution to this problem?