All Email Security Gateway models 600 and above, all firmware versions.
Email Security Gateway 600s (and above) have two NICs, and each can be configured with different IPs on separate networks. If so desired, you could use one Ethernet port to interface with the WAN portion of your network, and the other could then connect to your LAN. If set up in this fashion, the Email Security Gateway would act as the mail gateway for the LAN network.
To do this, first go to the Advanced > IP Configuration page of the Email Security Gateway's web interface. Then, under the Multiple IP Address Configuration (250 Max Per Interface) heading, add the IP address and subnet mask that you would like to assign to one of the two NICs (by specifying a broader netmask, you can assign a range of IPs to one of the NICs). You can select which network interface to use by selecting either 1 or 2 from the drop-down menu in the Network Interface column. Click the Add button to apply the new IP settings.
To control which interface is used when making connections to source IPs on either network, you can create static routes on the same page under the Static Routes heading. To do this, list each destination IP or subnet that should not use the Email Security Gateway's Default Gateway (configured on the Basic > IP Configuration page) under the Static Routes heading, and then specify which network Gateway Address to use for each.
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