You can check the list of the Registrants who received an email by creating a Registration History Report and filtering it by the Action and Message Name. 

Creating a Registration History Report

• Navigate to Event > Reports.
• Click Add Report to add a new Report.

• Enter a Report Name.
• Select Registration History as the Report Type.
• Click Save.

Filtering the Registration History Report to display the Email Action and Message Name

• Click the Filter tab on the left side menu.
• Under the Advanced Filters section at the bottom, click on the Columns drop down menu, and select Action.
• Set the Operator to (=).
• In the Value field, enter Email.
• Add another filter by selecting (and) from the drop down menu.
• Click on the Columns drop down menu, and select Message Name.
• Set the Operator to (=).
• In the Value field, enter the Message Name of the Email you sent. (This should be the name of the Email Template that was sent.)
• Click Save, then Run Report. This will generate a Report of the Registrants who received the Email.