Today I just has been alerted that there are too many ‘Relay access denied’ in my mail server. It is not normal. It must be any clients/workstations infected by viruses and try to relay to my mail server.
You have a Local Area Network (LAN) that using Linux as a router. In the Linux box there are two network interface card (NIC) attached to it, one has public IP that connect to Internet Service Provider (ISP) and the other one has private IP address that connect to the LAN.
In a Local Area Network (LAN), sometime you need to divide policy access based-on IP Address. For example, only specific IP addresses can access the web, email, FTP, facebook and so forth. Others will be dropped.