A computer network Firewall refers to a network device which blocks certain kinds of network traffic, forming a barrier between a trusted and an untrusted network.
Firewall is vital component in network administration and management.

For a complete seminar on Firewall, research on following points:
Packet filtering
Network security
Packet filtering
Port filtering
IP filtering
URL filtering
Application gateway
Circuit-level gateway
Network inbound and outbound communications
Proxy servers
Squid proxy
Network routers
Hardware Firewalls
Software firewall (VisNetic Firewall etc)
Cisco, Sonicwall, Netgear, D-Link

More related to email server, but have a look at it:
Composite Blocking List (CBL), DNS-based Blackhole List

External References
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Recommended technology reading: CRM Software