A method includes receiving, in a computer including a processor and memory, message reporting data indicating source addresses associated with email messages received by at least one receiving organization. Each source address specifies a sender of a selected one of the email messages. In the computer, outbound providers associated with the source addresses are identified. In the computer, the source addresses are classified based on the identified outbound providers. In the computer, the source addresses are aggregated based on the classification. In the computer, a report is generated including the aggregated source addresses.