There are probably different causes. I started getting these all day long after I disabled the default email app using:
I did that because I was disabling all built-in apps that I wasn't using to try to improve device performance. This is the only disable that caused any problems. The problem was solved when I re-enabled it. I did uncheck the "Show notifications" box without any problems, so at least I don't see its presence and have to deal with it, even though it is enabled to run in the background.