This recipe allows you to restrict partial user email addresses during checkout.
For example, you could specify that all @paidmembershipspro.com
email addresses are allowed to sign up—without having to enter each user’s full email address.

The Code Recipe (PHP)
Adding the Recipe to Your Website
You can add this recipe to your site by creating a custom plugin or using the Code Snippets plugin available for free in the WordPress repository. Read this companion article for step-by-step directions on either method.