Only allow users with an email address at a specific domain to register

limit_users_single_email_domain.php

function allow_single_email_domain( $user_login, $user_email, $errors ) {
    if ( strpos( $user_email, '@youralloweddomain.com' ) != -1 ) {
	return;
    } else {
	$errors->add( 'bad_email_domain', '<strong>ERROR</strong>: This email domain is not allowed.' );
    }
}
add_action( 'register_post', 'allow_single_email_domain', 10, 3 );