Fundamentals of Spam Filtering

What is a spam filter? How do spam filters work? How do spam filters decide if an email is spam? What signals do spam filters look at? Do spam filters learn over time? What’s the difference between spam filters and firewalls? What is a “filtering engine” vs a “filtering network”? What’s the difference between server-side and client-side filters? What’s the role of ISPs in filtering? What’s the difference between spam filtering and reputation systems? How do spam filters “learn”? What are heuristic rules in spam filtering? How do machine-learning models detect spam? What signals do filters analyze (header, content, behavior)? What’s the difference between global and user-level filtering? What is a “junk” folder vs “spam” folder? How does Gmail’s “Categories” tab fit into filtering?