An inbound mail gateway isn’t one filter making one decision — it’s a pipeline of modules, each making its own accept, reject, or quarantine call. Chapter 14 maps it module by module: connection-level checks (DNSBL, greylisting), authentication verification (SPF/DKIM/DMARC), reputation scoring, content and attachment scanning, and the final routing decision.
One distinction the chapter insists on: a basic spam filter and a full email security gateway are not the same thing. A gateway adds sandboxing, outbound data-loss prevention, click-time URL rewriting, and compliance-grade audit trails — genuinely different scope, not just a fancier spam filter.