Mail opens randomly while running other fullscreen app

Has anyone experiencing the Mail app opening when your macOS is running anything fullscreen? It causes the screen to be split in two. Side by side. For example, if you're watching Apple TV, the movie will be placed to the left hand side of the screen and the Mail messages will get all the other half. [ | ]

It is pretty annoying and I couldn't guess the exact reason for that.

Then I deleted my Gmail account from Mail > Accounts and let's see what happens. If it doesn't pops up again in the middle of the TV shows I'll reply to this question sharing the result.

Btw, my system is an iMac (late 2013) running the Catalina 10.15.