Maybe interesting for you too, I have been looking at this mail background activity for a few days now and this is my observation:It's really so irritating that i'm considering performing another clean install next week. I have like 12 hours a day background activity when it is completely turned off makes no sense.
I force restarted my phone and noticed that “Mail” stopped its background activity a few hours after I did that. When I force restarted again, another mail app I use named ”Spark” started with the same background activity in stead of “Mail”. Then after about 24 hours it just stopped and it’s been like that for over 2 days now.
Not sure what that means, but it seems like it’s related to the accounts that I have on the phone and maybe something like syncing with the server? I’m going to check what it does when I update to 16.4 when it’s out, I can imagine that it will try to do something like that again…we’ll see.