A spam gateway filters unsolicited emails at the server level, significantly reducing spam before it reaches user inboxes.
Why Not A (Load balancer): Load balancers distribute traffic but don't filter spam.
Why Not C (Mail forwarding): Forwarding doesn't filter spam.
Why Not D (Proxy servers): Proxies control web traffic, not email filtering.
CompTIA A+ Exam Reference: Core 2 (220-1102), Section 2.10, email security controls.