How to setup Email Security to use Round Robin or Fail-over when there is more than one downstream server.
Here there are two options one is:
If Round robin is specified, email will be load-balanced by sending a portion of the email flow through each of the servers specified in the text box in round-robin order. All of the servers will process email all the time.
If Fail over is specified, the first server listed will handle all email processing under normal operation. If the first server cannot be reached, email will be routed through the second server. If the second server cannot be reached, email will be routed through the third server, and so on.
These choices enable queuing, so if all of the specified downstream hosts are unavailable for any reason, email will be queued and will retry transmission later.