Change validation trigger for billing email field with Checkout for WooCommerce.

change-billing-email-validation-trigger.php

<?php

// Include in your theme's functions.php file or with your favorite snippets plugin
// Do NOT include the opening PHP tag above (<?php)

add_filter(
	'woocommerce_checkout_fields', function( $fields ) {
		// Change billing email validation trigger to focusout
		$fields['billing']['billing_email']['custom_attributes']['data-parsley-trigger'] = 'focusout';

		return $fields;
	}, 2000
);