Mail Is Putting Good eMails in My Junk Folder

Mail Is Putting Good eMails in My Junk Folder. I open the Junk folder and it contains a lot of files that are not junk. How does it decide (which it does, incorrectly) what is junk? My memory (in my head) tells me there is a "this is not junk" option, but apparently my memory is, as it all to often does, misleading me.