You can ensure that only approved emails are able to register for your event via the Restrictions section
Firstly you will need to have at least one ticket created for your event.
Note: These restrictions are on a per ticket basis, meaning that each ticket will need its own restrictions list added. If the restrictions section is left blank any email will be able to register for that ticket.
If you already have your ticket created, simply navigate to the Registration tab and then click Ticket and select the ticket you would like to add restrictions to.
Next, select the Restrictions button, a dialogue box will appear. Here you can paste in the approved emails that will be allowed to register for the event. Make sure there is only one email per line.
You can also include an entire domain here as well. So if you only want attendees using an @gmail.com email you would simply input "gmail.com" into the dialogue box.
This, for example, means that anyone trying to register with a Yahoo or Hotmail email address will be met with an error message that will instruct them to get in contact with the event organizers if they would like to register for your event.