Enables a debugger for the contact form 7 plugin that will add the error info to the AJAX response

debug-for-contact-form-7.php

<?php
/**
 * Plugin Name: Debug for Contact Form 7
 * Description: Enables a debugger for the contact form 7 plugin that will add the error info to the AJAX response
 * Version: 1.0.0
 * Author: Bernhard Kau
 * Author URI: https://kau-boys.de
 * License: GPLv3
 * License URI: http://www.gnu.org/licenses/gpl-3.0
 */

/**
 * @param array      $items     The items for the JSON response
 * @param array      $result    The result of the mail submission
 *
 * @global PHPMailer $phpmailer The global PHPMailer object
 *
 * @return array The items array
 */
function debug_for_contact_form_7_add_error_to_ajax_response( $items, $result ) {

	if ( 'mail_failed' == $result['status'] ) {
		global $phpmailer;
		$items['errorInfo'] = $phpmailer->ErrorInfo;
	}

	return $items;
}

add_action( 'wpcf7_ajax_json_echo', 'debug_for_contact_form_7_add_error_to_ajax_response', 10, 2 );

The status text for a spam report can also be "spam". Maybe an if ($result['status'] == "spam") would be useful as an addition.