This Variable will allow the POP3 Server to work in the Advanced Server configuration (partitioned server). It should be set to the IP address on which the server should listen.
Without this Variable on Partitioned Servers the POP3 server will not work, therefore, on partitioned servers where each server has a unique IP address. Sharing an IP address will not work (unless each server listens on a different port for POP3, which would mean using non-standard ports other than 110).