There are three built-in reports that may interest you in the APM: Exchange Reports report category in SAM. All three are web-based and editable. You can also schedule and export the reports.
Users By Mailbox Quota
Users By Mailbox Size
Users By Messages Sent This Week
Thank you for suggestion. However the Users by Messages Sent This Week will only show internal users and messages sent from within our Exchange environment. I need a report more in line with top message volume by sernders address regardless of internal or external. We have many back end systems which send emails thru our exchange server via relay/recieve connectors. From time to time our email infastructure gets bombarded by messages from one of those back end systems spitting out alerts/errors ect. So we need a top 10 volume of messages by address.