Email goes to spam folder

I have a family member with whom I've exchanged emails for years with no problems. Suddenly, about 3 weeks ago, all of his emails have gone into my Cox spam folder. Each time I've clicked on the "This is not spam" button that email has been moved back to my inbox. I've done this numerous times but all new email from him continue to go to the spam folder. I emphasize that I've received emails from this same person, same email address, for years with no problems until about 3 weeks ago. How do I correct this problem? If it means anything his email provider is tampabay.rr.com. Thanks for any help.