Email keeps going to spam folder

Important email from one person keeps going to my spam folder.  A whitelist would solve this, but there is no whitelist on Cox email.  I keep clicking "This is not spam," but apparently the system doesn't learn.  This is really irritating as I keep missing important emails.  I chatted with Cox for help and was told that the ONLY way to get this fixed was to sign up for Cox Care at an additional $10/month.  Unbelievable.  Right now, I have spam filtering turned off so I can get all my email, which also means actual spam gets dumped in my inbox.  I have no blacklist, no blocked senders, no spam rules that could be causing this.  Really unimpressed with Cox's lack of concern about this.