You do not need Message Networking to network multiple Messaging systems. You can use Message Networking as an LDAP server database to update the local and remote user database and to perform user lookup. Message Networking does not have any role in delivering the messages if messages are sent from Messaging to Messaging.
Message Networking is required when you network the messaging systems such as Audix, Aria, and other legacy systems to the Messaging system. To network with the traditional voice messaging systems, Messaging uses Message Networking. Message Networking not only updates the user database and performs the user lookup but also delivers the messages.
Messaging supports a maximum of 100,000 remote users, regardless of whether Message Networking is used to connect the Messaging system to other messaging systems.