In a multiple Exchange servers organization only servers you intend to report need to be licensed.
If you do not license a particular server, you will not be able to produce related reports.

• Servers generating message tracking files that will be imported by PROMODAG Reports.
• Server where IIS generates OWA logs that will be imported by PROMODAG Reports.
• Servers hosting mailboxes, for mailboxes and information stores content reporting.

Exchange 2007 server roles

Exchange 2007 introduces five roles to the Exchange organization: Edge Transport, Hub Transport, Client Access, Mailbox. The number of servers to be licensed depends on their respective role in the Exchange 2007 organization.

• Edge transport servers will generate message tracking files used for connector flow reports production.
• Hub transport servers will generate message tracking files both used for mailbox traffic and connector flow reports production.
• Mailbox servers will be used for mailbox storage and content reports production.
• Client Access servers will be used for OWA and Active Sync activity reports production.

•  Unified Messaging servers are not taken into account by Promodag Reports.

Clustered server

Only active nodes need to be licensed. You need, once the failover server wakes up, to unregister the failing server and register the failover server instead by using the registration window (Help > Registration).

Front-end and back-end servers

Usually, frond-end servers will forward email to servers hosting the mailbox store. In this case, the sent or received events are generated on the mailbox server whereas forward events are recorded on the front-end servers. These events are not used by PROMODAG Reports for producing traffic reports, but flow reports instead.
Therefore, for traffic reports, you need to analyze mailbox servers. For flow reports (Reports > Connector Flow menu),  you need to analyze front-end servers.