Often, Google Apps Admins will be asked to report on total email usage for their domain. The Gmail Aggregate Report is exactly what is needed to create this report; it generates a downloadable, day-by-day summary of emails sent and received by your domain.
To generate this report:
Get total emails by day from Google Apps Admin Reports
– In the Google Apps Admin console, click on Reports.
– Click the down arrow next to the Total emails header and choose your download method.
– The data will present a day-by-day breakdown of emails sent and received for your domain. Google seems to save roughly 6 months of data for this report, so I suggest setting a reminder to download it every quarter so that you can keep a historical record of your own.