What is the role of a spam filter in the mail flow?

A spam filter inspects incoming messages and classifies them based on trust, content, and reputation signals. It analyzes factors such as the sender's IP reputation, domain authentication results (SPF, DKIM, DMARC), message formatting, and even engagement history to decide whether a message belongs in the inbox or spam folder.

Over time, it adapts and learns from user behavior. When recipients mark legitimate messages as spam or rescue emails from the junk folder, these actions help train the system to make smarter decisions in the future.

It is like a customs checkpoint that inspects every shipment before allowing it into the harbor, separating legitimate cargo from contraband while improving its inspection process after each voyage.