A proxy firewall is the endpoint of the incoming and outgoing connection. It can perform extensive security and validation scans on the packets it processes. The proxy runs safe, uncorrupted, and bug-free versions of protocols and software.
Firewalls can enforce an organization's security policies by filtering the outgoing traffic of internal users to ensure that it complies with usage policies. They filter application layer messges to check for illegal application content. Application firewalls and packet filter firewalls are complementary in terms of what parts of a message they examine. Different application programs have different behaviors and therefore there must be a separate application proxy program for each application.
Firewalls do not protect everything
Firewalls do not protect against viruses, there can still be threads that viruses attach to e-mail and could get into the system.
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