You can't whitelist an address when it's the first message you've gotten from a person and it ends up in spam. Many of my real messages flagged as spam are the first time I've ever heard from a person. It's a public facing e-mail address that gets lots of random questions. 

Then there is the issue when SW doesn't follow the whitelist. I used to get messages from .gov addresses that got flagged as spam due to the discussion of animal reproduction even when the individual sender was shown in my whitelist! That's why I disabled spam checking, it was never accurate enough for my uses.