You can review the recipients for a sent message in two ways:
1. Tracking Results - click on the number that represents the recipients to which the message was sent (SENT: ALL)
2. From the Send Your Messages list:
Find the message and click the Edit icon
Scroll to the bottom of the Preview page, and click Next
On the Email Details page, scroll to the Who do you want to receive this email section. Expand all categories (Groups, Selected Contacts, etc.) to view recipients selected