The Admin Manual says that if you need to use a relay host, you shoud fill the "Relay host for messages leaving...
the local Internet domain" field with the machine host name. But, what if you need more than one relay host for load balancing for example?
You need to use the hosts names or IP addresses from the relay hosts separated by a comma. Ex.:smtp1.acme.com,smtp2.acme.com,smtp3.acme.com or 10.10.10.1, 10.10.10.2, 10.10.10.3