Hi,
I had this working, but after an update several months ago it has stopped. Deleted messages go to the deleted folder, but now are never are removed unless I open the folder and manually delete all.
Purge [Deleted] on exit is on.
"Back to close from message list" is on. (I have also tried turning this off.)
"Confirm moving to deleted" is off.
"After deleting a message" is "go back to message list.
Moto X 2015 running Android 5.1.
Thanks for any ideas!