How do I whitelist email addresses in Mail.app to prevent from being flagged as spam?

I have two questions that are closely related.

1. Clicking “Not Junk” on a message in the junk folder seems to have no effect at all.
   Is there an element I’m not understanding, or maybe I’m missing something all together?
   I’d like to figure out how to get “Not Junk” to work, or find a way to whitelist certain addresses.
   I’ve seen the method about adding a rule to move a message to another folder before junk can catch it, but it only works about 50% of the time, and regardless, that's such a hack; I’d like a more solid, reliable method, if one exists.
   
2. Why is it that some messages end up in my junk box, yet they’re not marked as junk?
   In other words, the message preview isn’t lighter text like it is for all other junk messages (it's solid black, like all regular messages), and when viewing the message, I don’t see the “Load Remote Content” and “Not Junk” buttons.
   It seems like Mail.app didn’t flag it as junk, yet for some reason it’s in my junk folder anyway.

Any insight on either of these questions is appreciated.

Thank you in advance.

I have two Mac's connecting to all my email (a 2006 Mac Pro with El Capitan, and a 2012 MacBook Pro with Mojave), also an iPhone 7 Plus, and two iPads, all the latest iOS version...incase that info is useful.