Look to manage this feature within the following admin tool:
REGISTRATION SETUP > ALLOW/BLOCK EMAIL DOMAINS
When you go to the above admin tool you'll see the following:
Within that screenshot you'll see:
With this option you have the ability to specify certain domains that you want to allow to register on your site. In other words, only users using email addresses from the particular domains you enter here will be able to register on your site. No use of "wildcards" is allowed within this feature. The below screenshot is an example of a configuration where you would only allow registrants that registered with an email from the following domain:
geodesicsolutions.com
With this option you have the ability to specify certain domains that you want to block from registering on your site. Users using email addresses from the particular domains entered into this admin tool will NOT be able to register on your site. No use of "wildcards" is allowed within this feature. The following is an example configuration where you would block registrations where the email domain is within:
gmail.com hotmail.com yahoo.com