Mail is good not bad.
It’s bad people trying to find an open relay or to hack into it to send spam through you.
Sometimes I get the notifications like bullets from a machine gun, one after the other on my phone.
One more thing. It's what you've said above. You're using an Android phone.
To the best of my understanding (I don't know much about Android os), it's not as restricted as iOS devices when it comes to background activities. iOS does not allow apps in the background to run but for a very short duration after you close them.
I believe that's not the case with Android, apps can run at will, and broadcast your location and activity 24/7 to Google, Facebook and the nasty Martian who almost wiped us off the face of the planet
The same goes for those Synology apps you have connecting to your NAS. They think they're doing the right thing. Maybe there is a way to limit their behavior.
if we don’t have the “login IP address” we can just speculate about a reason. And speculation will never support healt thinking (except long debate supported by fresh, cold beer). Next layer of the causality (OS,App,...) needs wait for the first stage.