The Internal vs External report shows the total number and size of messages that were sent within the organisation over a given date range. Switch between showing the number of emails, and the total size of the emails, through the buttons on the bottom left of the chart.
You can change the date range of the report through the Date Selection tool to the top right of the report. By default, it will show the last 30 days of data in a pie chart - You can change the chart type to a bar or line chart through the buttons to the bottom right of the chart.
The data table lists each date and the total number or size of emails sent internally within your organisation and externally outside your organisation. You can add, remove or re-order these columns to display additional information such as Office 365 Licence.
As with all of the reports, this information can be customized and scheduled.