To access your domain settings, click on the Settings button in the upper-right and select System Configuration.

Navigate to the Settings tab.

Scroll down until you see the Valid Email Domains and Invalid Email Domains settings. Enter the domains you want to allow/reject including the .com portion. For example, gmail.com, yahoo.com, or hotmail.com.

Using the Valid Email Domains setting allows you to specify which domains are allowed in your platform. This will effectively block all other domains that are not included. For example, adding only skyprep.com will prevent all non-skyprep.com domains from being used.

Invalid Email Domains is the reverse, with you specifying which domains are not allowed. Using this option will allow all other domains that are not included. For example, adding only skyprep.com will allow the use of gmail.com, yahoo.com, etc.

Once you've set up the domains to your preference, scroll to the bottom of the page and click Update Program Settings.