User Registration Plugin Whitelist.php

/* https://wordpress.org/plugins/user-registration/ */

function ur_validate_user_email( $single_form_field, $data, $filter_hook, $form_id ) {

	$value 	   = isset( $data->value ) ? $data->value : '';
	$whitelist = array( 'domain1.com', 'domain2.com', 'domain3.com' );
	$parts     = explode( '@', $value );

	if( ! in_array( $parts[1], $whitelist ) ) {

	    add_filter( $filter_hook, function ( $msg ) use ( $field_label ) {
	        return __( 'You may only register with an approved email address.', 'user-registration' );
	   });
	}
}
	
add_action( 'user_registration_validate_user_email','ur_validate_user_email', 10, 4 );