A loopback interface is a logical interface that is always up (no physical link dependency) and the attached subnet is always present in the routing table.

The FortiMail's loopback IP address does not depend on one specific external port, and is therefore possible to access it through several physical or VLAN interfaces. In the current release, you can only add one loopback interface on the FortiMail unit.

The loopback interface is useful when you use a layer 2 load balancer in front of several FortiMail units. In this case, you can set the FortiMail loopback interface’s IP address the same as the load balancer’s IP address and thus the FortiMail unit can pick up the traffic forwarded to it from the load balancer.

For information about adding a loopback interface, see “Configuring the network interfaces”.