How do spam filters decide if an email is spam?

Spam filters make decisions by combining multiple signals into a weighted calculation. No single factor determines the outcome. Instead, filters evaluate sender reputation, authentication results, content patterns, and recipient engagement to produce a confidence score.

The process begins before the message even arrives. During the SMTP connection, filters check whether the sending IP has a history of abuse, whether SPF and DKIM pass, and whether the domain has a clean record. If these early checks fail, the message may be rejected before content is ever analyzed.

Once accepted, the content itself is scanned for known spam patterns, suspicious links, and structural anomalies. Modern filters also consider how recipients have historically interacted with mail from that sender. High complaint rates and low engagement push messages toward the spam folder.

Think of it as a panel of judges scoring a vessel before it enters the harbor. Each judge evaluates a different aspect, and the combined score determines whether the ship docks at the inbox pier or gets redirected to quarantine.