Some people who have sent me emails for a long time have suddenly had their emails designated as spam. Using the 'more' tab, I unmark such emails which go into my Inbox - so far so good. But when I get emails later from the same address, they go back into spam again. The same happens in reverse - emails from advertisers that I don't want to hear from I mark as spam, but some of them (not all) appear again in my Inbox.
Is there a way to mark emails from specific senders permanently as spam or as not spam?