WooCommerce Mail Params: customize "To" recipient with billing firstname and lastname along to the email for better email delivrability

woocommerce-mail-callback-params.php

<?php
/**
 * WooCommerce: override mail params
 * @param          $params
 * @param WC_Email $email
 *
 * @return mixed
 */
public function custom_woocommerce_mail_callback_params( $params, \WC_Email $email ){

  $recipient = $params[0];
  $before_recipient = '';

  // Only do this if recipient is an email only, and just for customer emails (not admin emails)
  if ( is_email( $recipient ) && $email->object instanceof WC_Order && str_contains( $email->id, 'customer' ) ) {
    if ( method_exists( $email->object, 'get_billing_first_name' ) && method_exists( $email->object, 'get_billing_last_name' ) ) {
      $before_recipient = $email->object->get_billing_first_name() . ' ' . $email->object->get_billing_last_name();
    }
  }

  // Complete recipient if not empty
  if ( $before_recipient != '' ) {
    $params[0] = $before_recipient . ' <' . $recipient . '>';
  }
    

  return $params;
}

add_filter('woocommerce_mail_callback_params', 'custom_woocommerce_mail_callback_params', 20, 2);