More Info
Note: If you are receiving mail directly from the internet and you wish to configure an additional WorkgroupMail server as a backup domain server then refer to the following FAQ
Can I use WorkgroupMail as a backup MX server ?
Note: Your internal mail clients will be configured to communicate with a single mail server as specified by its computer name or IP address. To have a hot Backup mail server capable of dynamically taking over from a failed Primary would require some form of DNS load balancing. It would also need replication of data between the mail servers, unless both the primary mail server and the backup mail server were configured to access the same data across the network.
Detail
You can install WorkgroupMail (using the primary licence) on a backup machine that must be located within the same building and which must be used only as a backup server with a duplicate configuration. It will be a hot replacement for the primary mail server, should it go off-line. The primary copy and the Backup copy must not be used concurrently. If you require two WorkgroupMail servers to be running concurrently then they would require separate licences.