This question already has an answer here:
I have mail app open in background i.e. no window open. A new window randomly pops up without any conscious trigger by me. No new mail would have come.
Once it happened while I was in full screen mode of another app and the mail app split the window in half and got opened. This happens even when I am working in non Full-screen mode.
I presume that it has connection with notifications but other reasons are also welcome.
Once, it popped up above other apps, and some accounts were transitioning from offline to online state. I cannot confirm about Google account as one answer mentions.
Why might this happen but more importantly, how to stop this other than quitting it properly? I want to stop it from opening it altogether, not only in split view.
The answer to Mail opens a window randomly in split screen with my fullscreen window recommends to untick a Mail preference. Unfortunately I get the behaviour even thought the preference is unticked, so I look for other solutions.