An 'Email Activity by User' graph will give you insight into the email activity of your team.
1. Click Reports > New Reports....
2. Select Other Reports > User Emails > Create.
3. Change the Show drop down to All user emails.
4. If you wish, select the specific Date Range.
5. Drag the User field, Emails Received and Emails Sent fields into the Preview table.
6. Change the Tabular Format to Summary.
7. Click and drag the User header to create a Grouping.
8. Click the drop down arrow next to Emails Sent and Emails Received > Summarize this Field... > Sum > Apply.
9. Click Add Chart and select a Vertical Bar Chart.
10. Click Plot Additional Values and select Sum of Emails Received.
11. Click Formatting and give the Chart a title.
12. Click Save.
13. Click Save in the top right-hand corner, complete the fields and then click Save.
14. Add this Report to your Dashboard in Salesforce by following the steps in the below article: