Why are emails reloaded every time after reloading the Mail app in iOS9?

Question

iOS

Friends, tell me what's going on.
I connect the account, the POP protocol. It used to be like - when updating mail, the last 50 letters from the server were loaded. Now it loads everything that is in the folder on the server, but that's not so bad .... Worse, when I close the Mail application and open it again, it starts loading ALL the letters that are on the server again. If I exit and log into the application 5 times, then I will have 5 times the number of all the letters that are on the server.
How to solve this problem? I looked through all the settings, I did not find anything like that.
And one more problem: I select all the letters (mark a flag), click delete, he writes that it is impossible to delete them .... Well, how so ...
Update: the trash should have been created first by deleting one letter (very clever), and then everyone moves there. But here's another problem: I delete all letters from the trash, I exit the mail. I go again, and the letters are again all in the inbox .....

Answer 1

1) Install ios10
2) Check the login / password from the mail entered in the mail on the phone
3) Reboot the phone
There was a similar glitch, the problem was solved by the 2nd point (5S)