This can be found in the Gift Summary By Participant report.  Use the steps below to run properly:
  1. Go to Data Management > Reports > Report Writer Tab
  2. Click Create a New Report
  3. Select TeamRaiser in the Report Selection drop down
  4. Choose Gift Summary By Participant report
  5. In step 2. Select Columns, make sure to choose the Emails Sent column
  6. In step 5. Configure Filters, Select the TeamRaiser event you are reporting on
  7. Follow through the steps to finishing running the report. 
It can also be found in the TeamRaiser Participant Performance Report. Use the steps below to run this report:
  1. Go to Data Management > Reports > Report Classic Tab
  2. Click on TeamRaiser reports and select TeamRaiser Participant Performance Report
  3. Select options as desired, including filtering by event and click Submit Report
  4. Click the Disk Icon to save the Report.
The columns for Emails Sent will be in the downloaded report