Little Snitch - Little Snitch is a firewall application that monitors and controls outbound internet traffic.

Barracuda Email Security Gateway - Barracuda's Email Security Gateway is a powerful antivirus filter specifically designed to identify and eliminate email-borne threats, stopping hazards before they reach the network.

Gufw - Gufw is an easy, intuitive way to manage your Linux firewall.

Proofpoint Email Security and Protection - Learn about Proofpoint's email security solutions that protect organizations against threats such as email fraud, phishing, spam, bulk email, and viruses.

netfilter - Netfilter software comprises iptables, ipset, conntrack-tools, libnetfilter and more.

Cisco Email Security - Cisco Email Security protects against ransomware, business email compromise, spoofing, and phishing.