Mail deleting all emails from one sender

Mail is deleting all emails from one sender who is not blocked or marked spam. I've tried moving the emails out of the Trash into the Inbox and within seconds they are returned to the Trash. I've tried sending them to Junk and then using the Not Junk button to move them to the Inbox and within seconds they are returned to the Trash. I have checked the Blocked list and they aren't on it.