In 2026, spam filters from Gmail, Outlook, Yahoo, and Apple Mail use advanced AI and machine learning models that evaluate full context-not just individual "spammy" words-making sender reputation and engagement history crucial factors. Over-promotional, deceptive, and misleading subject lines (fake RE:/FWD:, exaggerated urgency, unrealistic offers) remain the fastest route to the spam folder. Engagement signals like opens, clicks, replies, and spam complaints now directly influence whether your future subject lines get filtered or delivered.
Anyone who has to know exactly what the rules are for inbox delivery is trying to figure out how close they can get to violating the rules without negative consequences. Senders that comply with the spirit of the rules don't care what the specifics of the rules are.
