I have a Samsung Galaxy S7 Edge running Android with all the latest updates. I use the default email app. About two months ago the way this app works changed. Here's how:
I have several different email accounts on different IMAP servers. These servers themselves handle spam in various ways: Server 1 moves it from the Inbox into a Spam folder (under the Inbox) on the server, and Server 2 adds "PROBABLE SPAM" to the subject line. I access these servers from both Thunderbird (on Microsoft Windows) and the default email app (on Android). Thunderbird has its own spam filtering (which moves messages from the Inbox to a local Spam directory), and the Android email app has never done any spam filtering.
About two months ago, I noticed that some messages on Server 2 (but not Server 1) were being moved from the Inbox folder of the server into separate Junk folder on the server (not under the Inbox). These messages were not actually spam, and had not been marked by the server's spam filter in any way. If I go into Thunderbird and move them back into the Inbox, a few seconds later they get moved back into the Junk folder. I checked with my email administrator and he confirmed that it is not the server doing this. So I turned off my phone and moved the messages back to the Inbox, and they stayed there. If I wait a few hours and then turn the phone on, the messages immediately move to Junk. So I know it is the phone doing the moving.
As I said, this only started two months ago; for years before this, my earlier S5 and my current S7 (which I got a year ago) did not do anything special with spam. I could not find anything in the app's settings which tells it to either do or not do something with spam. And as far as I know I do not have any other app on the phone that does anything with spam.
So, my question is: Why is the Android email app doing this, and how can I get it to stop? Thanks.