What does this report tell me?
The Top Email Senders Report displays a Count summary of all outbound e-mails sent by each of your users.

• Sender - This column shows the email Sender's address.
• Email Count - This column shows number of e-mails sent by the Sender.
• Percentage - This column shows the Percentage of total outbound e-mails sent by the Sender.

Top Email Senders Report - online Help

Traffic Classification Object (C and R) - online Help