Mail app opens randomly in MacOS 10.14.5 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.
This was happening to me for a while, and it became very annoying. I found that if I force closed it and turned the laptop off and on, it worked. If that continues to happen, turn off the WIFI connection and do the same steps. 

As to stop the behaviour in split mode, unchecking the checkbox in mail preferences for "Prefer split view while opening messages in full screen view" helps.