custom checkout filer

custom_checkout_filter.php

<?php 
function custom_add_validation_rules() {
	if ( ! isset( WC()->cart ) ) {
		return;
	}

    // Check if emails are ligitimate, if its not set add an error.
	if ( isset($_POST['additional_field_644']) && ! is_email( $_POST['additional_field_644'] ) ){
		wc_add_notice( 'Email Address For Summaries is not a valid email address.', 'error' );
	}
	if ( isset($_POST['additional_field_598']) && ! is_email( $_POST['additional_field_598'] ) ) {
		wc_add_notice( 'Email Address For Summaries (Retype) is not a valid email address.', 'error' );
	}
	if ( isset($_POST['additional_field_435']) && !is_email( $_POST['additional_field_435'] ) ) {
		wc_add_notice( 'Backup Email is not a valid email address.', 'error' );
	}
	if ( isset($_POST['additional_field_176']) && !is_email( $_POST['additional_field_176'] ) ) {
		wc_add_notice( 'Group Contact Email is not a valid email address.', 'error' );
	}
}
add_action( 'woocommerce_checkout_process', 'custom_add_validation_rules' );