What’s the difference between spam filters and firewalls?

Spam filters and firewalls serve different purposes at different layers of security. A firewall operates at the network level, controlling which traffic can enter or leave based on ports, protocols, and IP addresses. It does not understand email content.

A spam filter operates at the application level, specifically for email. It analyzes message headers, content, authentication records, and sender reputation to determine whether a message is legitimate, promotional, or spam. Firewalls let the traffic through; spam filters decide what to do with it once it arrives.

Enterprise environments typically use both. The firewall protects the network perimeter, while the spam filter (often part of an email gateway like Proofpoint or Mimecast) protects the inbox.

The firewall is the sea wall that blocks storms. The spam filter is the harbor master who inspects every vessel that makes it through.