1. As an Administrator navigate to Lists > Search > Saved Searches > New.
2. Select Customer.
3. Add a Search Title.
4. Criteria tab > Messages To : Recipient Email = is not empty.
5. Results tab > Messages To : Recipient Email.
6. Save & Run.
Results: A search that show Customers with emails that were sent out.
Note: User can add under Criteria e.g. Subject (Results tab > Messages Fields... > Subject).