For email(s) you just sent:
As soon as you click on send, a confirmation box will appear with text Success message. On the confirmation box, you will see a button which says View Report. Click on it and report will be downloaded to your local machine in Excel (.XLS) format.
For already sent email(s):
Please select the person to whom you sent the email and scroll down to the section called 'Participant administrator Emails'
Choose a report - Web Reports - 6.51 Participant Emails Report.