What is spam filtering (high-level)?

Spam filtering is the protective shield that examines every incoming message to decide whether it belongs in the inbox, the spam folder, or nowhere at all. It evaluates sender reputation, past engagement, authentication results, and content signals.

Behind the scenes, large providers like Gmail.com and Outlook.com operate machine learning systems trained on billions of messages. Early filters were simple keyword scanners looking for words like viagra or free, but modern systems rely on Bayesian filtering, sender IP reputation, and checks against RBLs (Real-time Blackhole Lists). These systems spot even subtle patterns such as timing irregularities, phrasing, or suspicious link behavior.

Spam filtering is the harbor’s defense fleet keeping the sea lanes clean.